I think she still has incredible sway with her supporters. I think most of them will come around of their own accord, but if I am wrong, she could bring most of the rest back. Also, she is very impressive for a VP candidate. She would almost surely outshine McCain's choice, and I don't think Republicans could necessarily mobilize around the idea of Clinton as VP as they could around the idea of Clinton as president.
Obama has a very good chance of losing the general if he does not pick Hillary, IMHO. There will be enough apathy to keep lost of Dems at home, particuarly the females.
But take into account that the Dems usually a good percentage of the black vote, Obama will capture 10% more. That in itself could be enough to win.