Supplement no. 8 to prospectus dated August 7, 2006, and
prospectus supplement dated August 7, 2006

Filed Pursuant to Rule 424(b)(7) of the Securities Act of 1933
relating to Registration No. 333-136361

1.50% Convertible Senior Notes due 2011
1.625% Convertible Senior Notes due 2013
Common Stock

     This supplement no. 8 to prospectus dated August 7, 2006 and prospectus supplement dated August 7, 2006 relates to the resale by certain selling securityholders of our 1.50% Convertible Senior Notes due 2011 and 1.625% Convertible Senior Notes due 2013, which we refer to as the notes, and the shares of our common stock issuable upon conversion of the notes.

     You should read this supplement no. 8 in conjunction with the following documents, which we refer to as the prior registration documents:

	•		the prospectus dated August 7, 2006, which we refer to as the prospectus;
	•		the prospectus supplement dated August 7, 2006, which we refer to as the prospectus supplement;
	•		supplement no. 1 dated August 18, 2006 to the prospectus and the prospectus supplement;
	•		supplement no. 2 dated September 7, 2006 to the prospectus and the prospectus supplement;
	•		supplement no. 3 dated September 29, 2006 to the prospectus and the prospectus supplement;
	•		supplement no. 4 dated October 23, 2006 to the prospectus and the prospectus supplement;
	•		supplement no. 5 dated November 22, 2006 to the prospectus and the prospectus supplement;
	•		supplement no. 6 dated December 22, 2006 to the prospectus and the prospectus supplement; and
	•		supplement no. 7 dated February 12, 2007 to the prospectus and the prospectus supplement.

     This supplement no. 8 is not complete without, and may not be delivered or used except in conjunction with, the prior registration documents, including any amendments or supplements to them. This supplement no. 8 is qualified by reference to the prior registration documents, except to the extent that the information provided by this supplement no. 8 supersedes or supplements certain information contained in the prior registration documents.

     Investing in the notes and the common stock issuable upon conversion of the notes involves risk. See “Risk Factors” beginning on page 4 of the prospectus supplement (as amended by the last two paragraphs of supplement no. 3 dated September 29, 2006 to the prospectus and the prospectus supplement).

     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 supplement no. 8 or the prior registration documents. Any representation to the contrary is a criminal offense.

     This supplement no. 8, together with the prior registration documents, constitutes the offer of up to $2,200,000,000 principal amount of our 1.50% Convertible Senior Notes due 2011, or 2011 Notes, $2,200,000,000 principal amount of our 1.625% Convertible Senior Notes due 2013, or 2013 Notes, and the shares of our common stock issuable upon conversion of those notes.

     The table under the caption “Selling Securityholders” beginning on page 43 of the prospectus supplement (as previously amended and supplemented by the tables under the captions “Additional Selling Securityholders” and “Revised Information Regarding Securityholders,” beginning on page 2 of each of supplements nos. 1 through 7 to the prospectus and prospectus supplement) is hereby:

	•		supplemented by adding the information regarding certain selling securityholders set forth in the table entitled “Additional Selling Securityholders” below; and
	•		amended by replacing the information in any of the prior registration documents regarding the selling securityholders identified in the table entitled “Revised Information Regarding Selling Securityholders” below with the information set forth in the table entitled “Revised Information Regarding Selling Securityholders” below.

     We prepared these tables based on information supplied to us by the selling securityholders named in the tables below on or prior to April 5, 2007. Information about the selling securityholders may change over time.

     We have assumed for purposes of the tables below that the selling securityholders will sell all of the notes and all of the common stock issuable upon conversion of the notes pursuant to this supplement no. 8 and the prior registration documents, and that any other shares of our common stock beneficially owned by the selling securityholders will continue to be beneficially owned.

     Except as set forth below, none of the selling securityholders has, or within the past three years has had, any position, office or other material relationship with us or any of our predecessors or affiliates.

     The selling securityholders identified below may have sold, transferred or otherwise disposed of, pursuant to transactions exempt from the registration requirements of the Securities Act of 1933, as amended, all or a portion of their notes since the date on which they provided the information regarding their notes.

Selling Securityholders

Additional Selling Securityholders

Number of

Principal Amount of

Shares of

Principal Amount of

2013 Notes

Common

2011 Notes Beneficially

Beneficially Owned

Number of

Stock

Owned and Offered

and Offered (USD)

Shares of

Beneficially

(USD) and Percentage

and Percentage of

Common

Owned after

Natural Person(s)

of 2011 Notes

2013 Notes

Stock Offered

the Offering

with Voting or

Name of Selling Securityholder (1)

Outstanding (%) (2)

Outstanding (%) (3)

(4) (5)

(6)

Investment Power

GLG Market Neutral Fund

28,000,000

1.27

—

—

498,716

107,104

(16)

Revised Information Regarding Selling Securityholders

																						Number of
										Principal Amount of												Shares of
		Principal Amount of								2013 Notes												Common
		2011 Notes Beneficially								Beneficially Owned								Number of				Stock
		Owned and Offered								and Offered (USD)								Shares of				Beneficially
		(USD) and Percentage								and Percentage of								Common				Owned after				Natural Person(s)
		of 2011 Notes								2013 Notes								Stock Offered				the Offering				with Voting or
Name of Selling Securityholder (1)		Outstanding (%) (2)								Outstanding (%) (3)								(4) (5)				(6)				Investment Power
Henderson Global Equity Multi-Strategy Master Fund Limited			5,550,000	(9)			*				—				—				98,852	(12)			—			Robert Villiers
JP Morgan Securities, Inc. (#) (17)			15,860,000				*				27,257,000	(11)			1.24				767,969	(13)			—				(7	)
Merrill, Lynch, Pierce, Fenner & Smith (#) (15)			95,665,000	(10)			4.35				42,398,000				1.93				2,459,081	(14)			—				(7	)

     Only selling securityholders that beneficially own the securities set forth opposite their respective names in the foregoing tables (including the tables included in the prospectus supplement and the supplements thereto) may sell such securities under the registration statement. Prior to any use of this supplement no. 8 in connection with an offering of the notes and/or the underlying common stock by any holder not identified above, the prospectus and prospectus supplement will be supplemented to set forth the name and other information about the selling securityholder intending to sell such notes and the underlying common stock.

The date of this supplement no. 8 is April 5, 2007