There seem to be at least two schools of thought on this.
One, is that it would be an obvious transition bring a longtime Clinton confidant on board and then easy Hillary into the VP spot.
The other is that Hillary harbors a great deal of animosity toward Ms. Solis Doyle (who while made several boneheaded moves, is often more vilified than the real architect of Hillary's strategy, Mark Penn) and this all but eliminates her from contention.
I, personally, think that Hillary still has a strong chance - if for no other reason due to the amount of support she earned and the promise of some of her supporters to tilt the election in McCain's favor.
Traditional wisdom would say Obama needs someone who balances his weaknesses. So a Joe Biden or someone with a strong grasp on foreign affairs. Could happen. I think it's more likely that Obama will pull a move from the Clinton/Gore 92 playbook and pick someone that doesn't really do much to balance the ticket, but rather re-enforces the mantra of change the campaign is trying to present.
I still say Biden or Webb are the strongest possibilities.