At this point I don't know who will be the Veep for either candidate, but I can tell you one thing, it won't be John Edwards for the Dems, since he just admited after denying it for so long, that he indeed had an affair.
I honestly don't see Hillary. She doesn't want to be anybody's VP. She wanted the top spot, although I think the combination of Obama/ Clinton would be a strong ticket indeed. Realistically it will probably be Bayh, Joe Biden or Kaine.
As for McCain, I don't think he gets along with Romney very well. I think Giulliani did a great job as Mayor of New York, and could pull a lot of votes from the center. He might not be a bad choice.
It'll be interesting.