LETTER AMENDMENT

Exhibit 10.1

EXECUTION COPY

LETTER AMENDMENT

Dated as of August 23, 2006

Ladies and Gentlemen:

We refer to the Five Year Credit Agreement dated as of June 6, 2006 (the “Credit Agreement”) among the undersigned and you. Capitalized terms not otherwise defined in this Letter Amendment have the same meanings as specified in the Credit Agreement.

It is hereby agreed by you and us that the Credit Agreement is, effective as of the date of this Letter Agreement, hereby amended as follows:

(a) The definition of “Responsible Officer “ in Section 1.01 of the Credit Agreement is amended in full to read as follows:

“Responsible Officer” means the chief executive officer, the president, the chief operating officer, the chief financial officer, the chief legal officer, the treasurer, the controller or an assistant treasurer of each of the Credit Parties and, for purposes of the definitions of “Competitive Bid Request” and “Request for Extensions of Credit”, such other officer as the Company may specify to the Administrative Agent in a written notice from time to time. Any document or certificate hereunder that is signed or executed by a Responsible Officer shall be conclusively presumed to have been authorized by all necessary corporate, partnership and/or other action on the part of the relevant Credit Party and such Responsible Officer shall be conclusively presumed to have acted on behalf of such Credit Party.

(b) Schedule 7.02 is hereby amended by deleting therefrom the figure “$500,000,000” and substituting therefor the figure “$1,500,000,000”.

(c) Schedule 10.02 is hereby amended in full to read as set forth as Annex A to this Letter Amendment.

The Company hereby certifies that, as of the date of this Letter Amendment, the representations and warranties contained in Article V of the Credit Agreement are correct in all material respects on and as of such date, with the same effect as if made on and as of such date, except where such representations and warranties expressly relate to an earlier date, and no event has occurred and is continuing that constitutes a Default or an Event of Default.

This Letter Amendment shall become effective as of the date first above written when, and only when, the Administrative Agent shall have received counterparts of this Letter Amendment executed by the undersigned and the Requisite Lenders or, as to any of the Lenders, advice satisfactory to the Administrative Agent that such Lender has executed this Letter Amendment. This Letter Amendment is subject to the provisions of Section 10.01 of the Credit Agreement.

On and after the effectiveness of this Letter Amendment, each reference in the Credit Agreement to “this Agreement”, “hereunder”, “hereof” or words of like import referring to the Credit Agreement, and each reference in the Notes and each of the other Loan Documents to “the Credit Agreement”, “thereunder”, “thereof” or words of like import referring to the Credit Agreement, shall mean and be a reference to the Credit Agreement, as amended by this Letter Amendment.

The Credit Agreement, the Notes and each of the other Loan Documents, as specifically amended by this Letter Amendment, are and shall continue to be in full force and effect and are hereby in all respects ratified and confirmed. The execution, delivery and effectiveness of this Letter Amendment shall not, except as expressly provided herein, operate as a waiver of any right, power or remedy of any Lender or the Administrative Agent under any of the Loan Documents, nor constitute a waiver of any provision of any of the Loan Documents.

If you agree to the terms and provisions hereof, please evidence such agreement by executing and returning at least two counterparts of this Letter Amendment to Susan L. Hobart, Shearman & Sterling LLP, 599 Lexington Avenue, New York, New York 10022.

This Letter Amendment may be executed in any number of counterparts and by different parties hereto in separate counterparts, each of which when so executed shall be deemed to be an original and all of which taken together shall constitute one and the same agreement. Delivery of an executed counterpart of a signature page to this Letter Amendment by telecopier shall be effective as delivery of a manually executed counterpart of this Letter Amendment.

This Letter Amendment shall be governed by, and construed in accordance with, the laws of the State of New York.

Very truly yours,

Agreed as of the date first above written:

as Administrative Agent and as a Lender

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ANNEX A

SCHEDULE 10.02

LENDING OFFICES AND NOTICE ADDRESSES

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