If he continues on the path he seems to be heading, then Biden. If he ends up having last minute doubts, then Bayh
I think I'm going to recant this earlier post. Two things that I think may end up hurting Biden's chances are that it seem as though him and Obama don't really have any personal chemistry and Obama wont be able to control Biden as Vice President. When it comes down to it, I think Obama will want someone who will be comfortable governing, as well as someone he will be able to control. Bayh and Kaine fit that profile
And given Kaine's own inexperience, I'm giving the edge to Bayh for the moment. My original prediction.