I second the thinking of Josh Patashnik over at TNR: the Democratic nominee shouldn't just pick a VP that helps win in 2008. He or she should pick a VP that can win in 2016, and that the Democratic Party sees as a future standard-bearer. As Patashnik notes, "One of the most important things a party does is cultivate talent for the future... There is no shortage of promising prospects who could achieve much if given that level of stature (the vice presidency)--why pass up the opportunity to put one of your rising stars in that position?"
I think that consideration effectively eliminates Sam Nunn, the oft-lionized foreign policy guru of the left who was born in 1938 and will be 78 in 2016. It probably also eliminates Joe Biden, who will be 74 in 2016, and Governor Ted Strickland of Ohio, who will be 75.