Prospectus Supplement No. 6   Filed Pursuant to Rule 424(b)(3) and Rule 424(b)(7)
(To Prospectus dated August 22, 2006)   Registration Number 333-136819
3.75% Convertible Subordinated Notes Due 2026
and the Common Stock Issuable Upon Conversion of the Notes
     This prospectus supplement relates to the resale by selling security holders of our 3.75% Convertible Subordinated Notes Due 2026 and the shares of our common stock issuable upon conversion of the notes.
     This prospectus supplement should be read in conjunction with, and may not be delivered or utilized without, the original prospectus dated August 22, 2006, Supplement No. 1 to such prospectus dated August 31, 2006, Supplement No. 2 to such prospectus dated September 14, 2006, Supplement No. 3 to such prospectus dated September 28, 2006, Supplement No. 4 to such prospectus dated November 1, 2006 and Supplement No. 5 to such prospectus dated January 11, 2007. The terms of the notes are set forth in the original prospectus.
Investing in these securities 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 the notes or the shares of common stock issuable upon conversion of the notes or determined if the prospectus or this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.
The date of this prospectus supplement is January 31, 2007.



     The information in the table appearing under the heading “Selling Security Holders” in the prospectus is amended by adding the information below with respect to persons not previously listed in the prospectus and by supplementing the information with respect to other persons previously listed in the prospectus that are listed below. This information was furnished to us by the selling security holders listed below on or before January 30, 2007.
     Based on information provided by the selling security holders, none of the selling security holders has held any position or office or has had any material relationship with us within the past three years.
                    Shares of           Shares of    
    Principal           Common           Common    
    Amount of           Stock           Stock    
    Notes           Beneficially   Shares of   Beneficially   Percentage
    Beneficially   Percentage   Owned   Common   Owned   of Common
    Owned and   of Notes   Prior to   Stock   After   Stock
Name   Offered   Outstanding   Offering   Offered   Offering   Outstanding
CBI Pension(1)
  $ 35,000       *       1,562       1,562              
Celanese Americas Corp Retire Pension(1)
  $ 250,000       *       11,156       11,156              
Cervantes Portfolio LLC(1)
  $ 300,000       *       13,387       13,387              
Earlham College Endowment(1)
  $ 150,000       *       6,694       6,694              
Fairfax County Employees’ Retirement(1)
  $ 300,000       *       13,387       13,387              
Kellogg Co. — Master Retirement Trust(1)
  $ 500,000       *       22,312       22,312              
Kellogg Co. — Welfare Benefit Trust(1)
  $ 200,000       *       8,925       8,925              
Pension Reserves Investment Trust Fund(1)
  $ 2,000,000       1.39 %     89,246       89,246              
Praxair Retirement Program(1)
  $ 190,000       *       8,479       8,479              
Reform Pension Board(1)
  $ 175,000       *       7,810       7,810              
RI Employees’ Retirement System(1)
  $ 250,000       *       11,156       11,156              
Rockwell Automation Master Trust(1)
  $ 175,000       *       7,810       7,810              
Southwest Carpenters Pension Trust(1)
  $ 300,000       *       13,387       13,387              
Teachers Retirement System of Louisiana(1)
  $ 1,000,000       *       44,623       44,623              
University of Southern California(1)
  $ 225,000       *       10,041       10,041              
Wyeth Retirement Plan-U.S. Master Trust(1)
  $ 500,000       *       22,312       22,312              
*   Less than one percent
(1)   Shenkman Capital Management, Inc. acts as investment manager to this security holder and as such has power to direct the voting and disposition of securities held by this security holder. Mark Shenkman is the chief investment officer of Shenkman Capital Management, Inc. and has oversight authority over all portfolio managers at Shenkman Capital Management, Inc.