Filed Pursuant to Rule 424(b)(3)

Registration No. 333-140208

PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 25, 2007)

$160,000,000

1.50% Senior Convertible Debentures due 2025

This prospectus supplement supplements and amends the prospectus dated January 25, 2007, relating to $160.0 million aggregate principal amount of our 1.50% Senior Convertible Debentures due 2025 and shares of our Class A common stock into which the Debentures are convertible. You should read this prospectus supplement in conjunction with the prospectus and this prospectus supplement is qualified by reference to such prospectus, except to the extent that the information in this prospectus supplement supersedes the information contained in such prospectus. This prospectus supplement is not complete without, and may not be delivered or utilized except in connection with, the prospectus.

The table beginning on page 27 of the prospectus sets forth information with respect to the selling securityholders and the respective amounts of Debentures and Class A common stock into which the Debentures are convertible that are beneficially owned by each selling securityholder which may be offered pursuant to the prospectus. The table is hereby supplemented and amended by adding the information below with respect to selling securityholders not previously listed in the prospectus and by superceding the information with respect to certain securityholders listed in the prospectus. We have prepared the table below based on information given to us by the selling securityholders named below.

	Principal Amount
	of Debentures		Percentage of			Number of Shares		Percentage of
	Beneficially Owned		Debentures			of Class A Common		Class A Common
Name of Selling Securityholder	That May Be Sold		Outstanding			That May be Sold		Outstanding
BNY Capital Markets, Inc.(24)	$	65,000		*			6,309		*
Henderson Global Equity Multi-Strategy Master Fund(25)	$	1,506,000		*			146,185		*
Henderson North American Equity Multi-Strategy Master Fund(25)	$	364,000		*			35,333		*
J.P. Morgan Securities Inc.(26)	$	38,000		*			3,689		*
Lehman Brothers Inc.(14)	$	4,500,000		2.81	%		436,808		*
Met Investor Series Trust — Bond Debenture(27)	$	900,000		*			87,362		*
Radcliffe SPC, Ltd. for and on behalf of the Class A Convertible Crossover Segregated Portfolio(18)	$	8,950,000		5.59	%		868,763		*

(24)	BNY Capital Markets, Inc., a registered broker-dealer, is a subsidiary of The Bank of New York, a publicly held entity.
(25)	The selling securityholders are each registered as a regulated mutual fund with the Cayman Islands Monetary Authority under Section 4(3) of the Mutual Funds Law of the Cayman Islands.
(26)	J.P. Morgan Securities Inc., a registered broker-dealer, is a publicly held entity.
(27)	Maren Lindstrom, on behalf of Lord Abbett & Co., has voting and investment power over the securities identified above as beneficially owned by Met Investor Series Trust — Bond Debenture.

Investing in the Debentures or Class A common stock involves risks. See “Risk Factors” beginning on page 10 of the prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is May 16, 2007.