This excerpt taken from the SIRI 10-K filed Mar 1, 2007.
Withholding. (i) A Participant may be required to pay to the Company or any Affiliate and the Company or any Affiliate shall have the right and is hereby authorized to withhold from any Award, from any payment due or transfer made under any Award or under the Plan or from any compensation or other amount owing to a Participant the amount (in cash, Shares, other securities, other Awards or other property) of any applicable withholding taxes in respect of an Award, its exercise, or any payment or transfer under an Award or under the Plan and to take such other action as may be necessary in the opinion of the Company to satisfy all obligations for the payment of such taxes. The Committee may provide for additional cash payments to holders of Awards to defray or offset any tax arising from the grant, vesting, exercise or payments of any Award.
(ii) Without limiting the generality of clause (i) above, a Participant may satisfy, in whole or in part, the foregoing withholding liability by delivery of Shares owned by the Participant (which are not subject to any pledge or other security interest and which have been owned by the Participant for at least six months) with a Fair Market Value equal to such withholding liability or by having the Company withhold from the number of Shares otherwise issuable pursuant to the exercise of the option a number of Shares with a Fair Market Value equal to such withholding liability.
(iii) Notwithstanding any provision of this Plan to the contrary, in connection with the transfer of an Option to a Permitted Transferee pursuant to Section 14(a), the Grantee shall remain liable for any withholding taxes required to be withheld upon the exercise of such Option by the Permitted Transferee.