This excerpt taken from the DIS 8-K filed Mar 14, 2008.
Under section 3406(a)(1) of the Internal Revenue Code, the Paying Agent making payment of interest or principal on securities may be obligated to withhold a percentage of the payment to a holder who has failed to furnish the Registrar with a valid taxpayer identification number, certification that the number supplied is correct, and that the holder is not subject to backup withholding. Holders of the bonds who wish to avoid the application of these provisions should submit either a completed IRS (Internal Revenue Service) Form W-9 (use only if the holder is a U.S. person, including a resident alien), or the appropriate form W-8 (use only if you are neither a U.S. person or a resident alien), when presenting the bonds for payment. See IRS Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities. Publication 515 and W-8 forms and instructions are available through the IRS via their web site at www.irs.gov.