So President Bush reversed a presidential pardon. But can he actually do that?

Pardon Power, a blog dedicated to, what else?, "the very latest news regarding presidential pardons and the pardon power (or clemency powers) as exercised in each state" is quite sure he can:
The fact that "the president" - in this case - meant two different presidents (Johnson and Grant), and the fact that - in this case - the warden had actually received the pardons but simply stuck them in his desk for a while, did not matter. The pardons had not actually been placed in the hands of Moses and Jacob DePuy, so the two men stayed in prison and were pardoned (by Grant) later.

TPM isn't so sure, though:
When asked by Rep. Hostettler (R) whether President Bush couldn't undo Clinton's pardons under the Du Puy case, she [sic] seemed to say that Du Puy had bee [sic] superseded in this regard by Biddle v. Perovich from 1927...

Any of our more legal-minded readers care to chime in?