Pay no attention to the supposed "short lists" being passed around. The purveyors of conventional wisdom usually blow this call. In fact, one could argue that hype actually diminishes a person's chances of being picked because, optimally, you want people to be both happy with the selection and surprised by it. It makes a bigger "splash" that way.
If the Obama campaign knows what it's doing, they'll find someone with "gravitas" who is affiliated in the popular mind neither with Washington nor with the Democratic party proper. People associate Tom Daschle, Hillary Clinton, Chris Dodd, and Joe Biden with both.