Noam linked to a letter Biden wrote in defense of Obama--and then suggested that he’d have some definite plusses as Obama's running mate. In the comments, BHLnyc picked up on Noam’s point and pointed to reasons why Biden may in fact be the best choice.
I hold to the belief that Biden is Obama's best choice for vice president.
As you've already stated, he's kind of a working stiff, which gives him credibility with Joe Lunchbucket, and he's a foreign policy and homeland security heavyweight, which gives him credibility with Elites. He's a respected, "seasoned hand," which could reassure seniors, and yet his shoot-from-the-lip candor might play well with younger voters.
He also happens to be blessed with a razor sharp mind that could be very effective in debates. (Certainly Rudy Giuliani found that out the hard way.)
I think it really comes down to whether Obama feels any chemistry with the guy. If so, Biden really should be at the top of his list.