My thoughts as of now (all this could change quickly):

If Hillary wins, there are all sorts of practical problems with Obama as both running mate and VP if she were to win. I don't know if Obama would accept if offered--he would be under a lot of pressure from many of his supporters to decline.
Still, the genie is out of the bottle with his candidacy. There is too much new energy his candidacy has brought into the process and if most or all of that is lost dissipates in a sea of disillusionment I think it is likely to be even more difficult for Hillary to beat McCain. If she wins she will energize the Republican base and the only way right now that I can see our side overcoming that and Hillary's negatives is to have Obama on the ticket--and, not just to have him on the ticket, but to have the two of them develop some good chemistry quickly and play off one another's strengths with confidence-inspiring ease.

There is a plus case for him to accept if offered: he won't get much blame if the ticket loses and he'd be next in line, presumptively. If she wins, he would gain high level experience, addressing a concern many have about his candidacy. For Hillary and her people it might come down to a choice between picking her poison: would she rather be overshadowed or outshined, or lose? My bet would be the former. Together, they might conceivably be formidable, although, again, there are all sorts of bad chemistry and other trainwreck scenarios one can conjure.

If Obama wins, my first thought was Richardson. Richardson has substantial foreign policy experience and cred, he could help with the crucial Latino vote, he comes from a subregion in which Dems are competitive and from a state that is normally in play, beyond foreign policy experience he has a great deal of governing experience that should be a source of reassurance. On the down side, he is an awful campaigner. And he might not help enough with working class voters, clearly a major issue with Obama's candidacy.

If Obama wins, I thought maybe Wes Clark as another possibility. Beyond the fact of his close ties with the Clintons, I think we might need him as SecDefense if we win. But if necessary to win, yes, maybe have him on the ticket.

Joe Biden is another possibility if Obama wins. Again, foreign policy experience. He would also be a logical short-lister for Secretary of State if Obama likes him.

If Obama wins Hillary will be the key player on health care in the Senate. They'll need to make sure she's on the right Committee or Committees. If anyone would be able to figure out a way to thread that needle and get a bill workable as policy through, it's her (or maybe Kennedy). If she were to lose, go back to the Senate with dignity and class, do what Hillary does and work her ass off, to help get health care, that could prove decisive if an Obama win brought with it as well a more favorable makeup in Congress. Lots of ifs, of course.

As I say, early thoughts...