Rasmussen has a video commentary today which says that the VP spot is Hillary's if she wants it because she has the votes at the convention to make it happen.
http://www.rasmussenreports.com/public_content/politics/election_20082/2008_presidential_election/daily_presidential_tracking_poll (see link in discussion)
Is that how the VP nominating process works- is it subject to a vote at the convention? If so, then Rasmussen is likely correct- Hillary's supporters will all vote for her, and she will only need something like 10% of Obama delegates to vote with them to have a majority for VP.
So all our discussion of whom Obama should pick may be based on a faulty premise - it may be in Hillary's hands more than Obama's.