Subsequent presidents can "un-pardon" people pardoned by previous presidents, so I have a hard time imagining that a commutation is somehow immune to that kind of thing.

EDIT: See follow-ups. I may be mistaken here, and need to run to a meeting, so I can't dig any more now.

EDIT 2: http://swampland.time.com/2008/12/25/more-on-pardons/

Relevant quote, "Ulysses S. Grant’s first clemency decision, on his third day in office, was to revoke two pardons granted by Andrew Johnson. Both men challenged Grant’s power to do so, and lost their case in federal court."

EDIT 3: Here's the link to the blog post linked from the Time article, in edit 2, from the Wayback Machine, for sake of completeness.

http://web.archive.org/web/20090212091707/http://pardonpower...