I know a lot of Democrats considered the Obama/Clinton ticket the "Dream Ticket", and unbeatable. The fact of the matter is, putting Hillary on the ticket would have been the one thing that would have united the unhappy Republicans on the fence, and rallied them behind McCain. That's the last thing Obama needs in a so far close race. I still think Obama is going to win fairly easily, but putting Hillary on the ticket was very risky. Biden is the safest choice, which is the smart move when you're still the favorite to win.