Will he want to run for VP a second time?
And will Obama want him? Obama wanted Biden last time to alleviate concerns about his experience, a problem that Obama won't have next time.
Then...If Biden is off the ticket, does Obama make history with Hillary or Sebelius?
If Obama's a shoe-in for 2012, does he try to pick a running mate who would be poised to compete in 2016, like Warner or Schweitzer?
If Obama's in trouble, does he make a geographical pick like one of the Virginia boys or Bill Nelson? A demographic pick, an issue-based pick?
The running mate is unlikely to be a 2008 contender, if it isn't Clinton. The Class of 2008, in addition to Obama, Biden and Clinton, consists of Edwards and Richardson who have ethical issues, Bayh who pissed off the Democratic base, Kucinich who is just a little too Peewee Herman for the vice presidency, Gravel who is just plain nuts and would be in his 80s, Dodd who would probably prefer to stay in the Senate, and Vilsack who campaigned for Hillary and is deadly dull.