I think Senator Jay Rockefeller would be the ideal choice for Obama's Vice President. He's been a Governor for two terms, and a Senator since 1985 after that. True, he is 71 years-old, but if John McCain ends up being the Republican nominee I don't think that baggage is too heavy to bear. Rockefeller comes from West Virginia - a key state in the 2008 campaign and one it would be good for the Democrats to solidify. Also, if he is chosen, there is a Democratic Governor waiting to appoint his replacement and a deep Democratic bench in the state. Obama/Rockefeller has a nice ring to it.
However, Rockefeller is that great of a Senator. He frequently misses votes, and as far as I know there has not been any major legislation associated with him.