Yahoo has a good article this morning on potential VP picks that does something most such articles don't - it lists their ages:
* Joseph Biden, 65
* Wesley Clark, 63
* Hillary Clinton, 60
* Chris Dodd, 64
* Chuck Hagel, 61
* Tim Kaine, 50
* Sam Nunn, 69
* Ed Rendell, 64
* Bill Richardson, 60
* Kathleen Sebelius, 60
* Ted Strickland, 66
* Jim Webb, 62
Notice something? Kaine is the only one under 60 - in fact, he's only 50.
To me, probably the most important factor in picking a VP is picking someone who can be a strong candidate 8 years from now, since VPs are nominees so often. If Obama wants to help make lasting changes in this country, he will understand the importance of picking someone who will have a good shot at keeping his policies going 8 years beyond his own term.
Kaine is the only one under 60 (he's only 50) and was also a very early Obama backer when it wasn't fashionable, good soldier, and Governor of a major swing state who is term limited. He's demonstrated how Dems can talk effectively on how faith informs their progressive policies and values (re: death penalty).
I think it will, and should, be Kaine.