I think Joe Biden would be an incredibly interesting add to the Democratic ticket as Obama's VP choice. Often I hear that one of his biggest pros, that he strongly and clearly states positions, isn't afraid to go on the attack, and gets out in front of issues is also a downside when he says off the cuff things and grabs lots of negative press in the process. That said, I think his national security credentials are pretty great. I think his resume in general is impressive, and I think he would just make for a cool and strong addition to an already strong ticket. He's articulate, smart, and he's been around Washington for long enough to know the game, but still comes off as somebody who's not simply a player, someone who has a stance and sticks to his guns.
That's all pretty first-blush stuff, but I personally think Biden is one of the strongest possibilities, to my mind much stronger that Bayh or Kaine. I think there are people out there who have questions about experience and Biden could help sure up some of those folks, which is a good thing (although honestly I think their concerns are a little silly... still I think it's a real group in the electorate).
Any thoughts on Wes Clark or Kathleen Sebelius? I got the impression Clark was out of consideration for some reason, but saw some discussion of him on HuffPo today, and Sebelius is always mentioned but I rarely hear people go into much depth on the possibility.
I dropped this into comments too, but Purple State has some nice coverage of the Biden possibility here.