An interesting dustup from Bush's latest pardon attempts - after it was publicly revealed that one recipient's father gave almost $30,000 to the Republican party, Bush is trying to take the pardon back, and one 140-year-old case suggests he might be able to do it. If the pardon has been issued but not delivered and accepted, then it remains revocable.
I've been wondering whether Bush will try and push the envelope of the pardon power next month, issuing a blanket pardon without names attached to anyone involved in any capacity with extraordinary rendition and authorized interrogation techniques. I've thought he'd do it without naming names not just to exempt as many underlings as possible, but also in order to pardon himself and Cheney without doing so expressly. The problem comes with the old case at the link above, that "A pardon is a deed, to the validity of which delivery is essential, and delivery is not complete without acceptance, and a pardon by an outgoing President may be revoked by his successor before delivery."
I'm not sure if the concepts of delivery and acceptance still operate today, or how they worked for Carter's blanket pardon of draft evaders. To the extent they still work though and allow Bush to revoke his pardon, Obama could do the same thing.
Something Obama may have to think about.
UPDATE: See the comments, and also here for whether Bush really can revoke the pardon. I think that insofar as the legal question is open, Bush would have estoppel problems if he claimed that Obama couldn't revoke pardons that Bush himself gave but had not "delivered." I also think Obama would likely duck the issue, though.