I think that the prediction markets are treating the Biden choice as generally positive for Obama. Presidential future’s trader’s reacted positively right away, backed off a little, digesting the negatives (he said tough things about Obama during the primary; he has a serious case of hoof in mouth disease; he’s pretty liberal, etc.), and concluded that Biden would, on balance help Obama. He’s already ahead, so he falls on the ball during the last play. Not exactly bold change that we can believe in, but it will probably help him. Yes, Hillary would have been a slam dunk, but traders knew he wasn’t going to choose Hillary.
Biden was the best that Obama buyers could hope for given the fact that he would not countenance a real rival on the ticket. He may have announced in Springfield, Illinois, but Obama didn’t pull a Lincoln. There is no team of rivals.