<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>076 - Statement - Expected Future Principal Payments as of September 30, 2009 (Detail)</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption>In Millions</RoundingOption><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>{us-gaap_LongtermDebtTypeAxis} : Repayments OfPrincipal InNext Twelve Months [Member]
9/30/2011
USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>c279_AsOf30Sep2011_RepaymentsOfPrincipalInNextTwelveMonthsMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000353944</EntityValue><PeriodDisplayName /><PeriodType>instant</PeriodType><PeriodStartDate>2011-09-30T00:00:00</PeriodStartDate><PeriodEndDate>0001-01-01T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Repayments OfPrincipal InNext Twelve Months [Member]</ValueName><ValueType>us-gaap_LongtermDebtTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>igt_RepaymentsOfPrincipalInNextTwelveMonthsMember</Id><dimensionId>us-gaap_LongtermDebtTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>usd</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="Sep. 30, 2011" /><Label Id="2" Label="Repayments OfPrincipal InNext Twelve Months [Member]" /></Labels></Column><Column><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>{us-gaap_LongtermDebtTypeAxis} : Repayments OfPrincipal InYear Two [Member]
9/30/2012
USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>c280_AsOf30Sep2012_RepaymentsOfPrincipalInYearTwoMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000353944</EntityValue><PeriodDisplayName /><PeriodType>instant</PeriodType><PeriodStartDate>2012-09-30T00:00:00</PeriodStartDate><PeriodEndDate>0001-01-01T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Repayments OfPrincipal InYear Two [Member]</ValueName><ValueType>us-gaap_LongtermDebtTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>igt_RepaymentsOfPrincipalInYearTwoMember</Id><dimensionId>us-gaap_LongtermDebtTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>usd</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="Sep. 30, 2012" /><Label Id="2" Label="Repayments OfPrincipal InYear Two [Member]" /></Labels></Column><Column><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>{us-gaap_LongtermDebtTypeAxis} : Repayments OfPrincipal InYear Four [Member]
9/30/2014
USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>c281_AsOf30Sep2014_RepaymentsOfPrincipalInYearFourMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000353944</EntityValue><PeriodDisplayName /><PeriodType>instant</PeriodType><PeriodStartDate>2014-09-30T00:00:00</PeriodStartDate><PeriodEndDate>0001-01-01T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Repayments OfPrincipal InYear Four [Member]</ValueName><ValueType>us-gaap_LongtermDebtTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>igt_RepaymentsOfPrincipalInYearFourMember</Id><dimensionId>us-gaap_LongtermDebtTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>usd</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="Sep. 30, 2014" /><Label Id="2" Label="Repayments OfPrincipal InYear Four [Member]" /></Labels></Column><Column><Id>4</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>{us-gaap_LongtermDebtTypeAxis} : Repayments OfPrincipal After Year Five [Member]
9/30/2010
USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>c282_AsOf30Sep2010_RepaymentsOfPrincipalAfterYearFiveMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000353944</EntityValue><PeriodDisplayName /><PeriodType>instant</PeriodType><PeriodStartDate>2010-09-30T00:00:00</PeriodStartDate><PeriodEndDate>0001-01-01T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Repayments OfPrincipal After Year Five [Member]</ValueName><ValueType>us-gaap_LongtermDebtTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>igt_RepaymentsOfPrincipalAfterYearFiveMember</Id><dimensionId>us-gaap_LongtermDebtTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>usd</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="Sep. 30, 2010" /><Label Id="2" Label="Repayments OfPrincipal After Year Five [Member]" /></Labels></Column><Column><Id>5</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>{us-gaap_LongtermDebtTypeAxis} : Repayments OfPrincipal Total [Member]
9/30/2010
USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>c283_AsOf30Sep2010_RepaymentsOfPrincipalTotalMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000353944</EntityValue><PeriodDisplayName /><PeriodType>instant</PeriodType><PeriodStartDate>2010-09-30T00:00:00</PeriodStartDate><PeriodEndDate>0001-01-01T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Repayments OfPrincipal Total [Member]</ValueName><ValueType>us-gaap_LongtermDebtTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>igt_RepaymentsOfPrincipalTotalMember</Id><dimensionId>us-gaap_LongtermDebtTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>usd</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="Sep. 30, 2010" /><Label Id="2" Label="Repayments OfPrincipal Total [Member]" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>igt_PrincipalPayments</ElementName><ElementPrefix>igt</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>instant</PeriodType><ShortDefinition>Amount of long-term debt maturing within a period of time following the date of the latest balance sheet presented in the...</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>label</PreferredLabelRole><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>15600000</NumericAmount><RoundedNumericAmount>15.6</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>true</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios></Cell><Cell><Id>2</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>84400000</NumericAmount><RoundedNumericAmount>84.4</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>true</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios></Cell><Cell><Id>3</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>850000000</NumericAmount><RoundedNumericAmount>850.0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>true</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios></Cell><Cell><Id>4</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>800000000</NumericAmount><RoundedNumericAmount>800.0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>true</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios></Cell><Cell><Id>5</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>1750000000</NumericAmount><RoundedNumericAmount>1750.0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>true</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Monetary</Unit><ElementDataType>xbrli:monetaryItemType</ElementDataType><SimpleDataType>monetary</SimpleDataType><ElementDefenition>Amount of long-term debt maturing within a period of time following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates.</ElementDefenition><ElementReferences>No authoritative reference available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Principal payments</Label></Row></Rows><Footnotes /><NumberOfCols>5</NumberOfCols><NumberOfRows>1</NumberOfRows><ReportName>Expected Future Principal Payments as of September 30, 2009 (Detail) (USD $)</ReportName><MonetaryRoundingLevel>HundredThousands</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>false</HasCustomUnits><SharesShouldBeRounded>true</SharesShouldBeRounded></InstanceReport>
