Biden, Bayh, or Kaine
Last night on Countdown, Howard Fineman mentioned that he had "insider information" that Obama's shortlist is down to three names: Senator Evan Bayh, Senator Joe Biden, and Virginia Governor Tim Kaine.
Of these three names, I think that Tim Kaine is the best bet. He's a governor, which adds executive experience to the ticket. He's from an increasingly "purple" state, which, with his help, might turn blue in November. From what I've seen, he's a relaxed, charismatic campaigner, which is never a bad thing. The only problem with Kaine is that he has only been governor for one term, so that doesn't help Obama with the "lack of experience" perception that has dogged his campaign.
I don't know a whole lot about Evan Bayh, but my understanding is that he's pretty conservative, which might piss off Obama's activist base and make some of them stay home. Biden will be dogged by plaigarism charges from twenty years ago, so although he's a great Senator, he's probably out.