Clinton/Clark - Uh, Clark didn't do so hot his last time out (poor showings in '04), so I'm not sure he's really going to help. All most people know is he looks like somebody's nice Grandfather. He's also got a penchant for sticking his foot in his mouth, and with Bill already planting both feet in his, a ve-ep who does the same probably isn't the best idea.
Obama/Webb - In one fell swoop, this neutralizes almost all of John McCain's strengths and sticks a fork in the eye of the NRA (since Webb packs heat in the Senate after all). I don't know how Webb will go over with progressives, but they might give him a pass because he does strengthen the ticket. Could people really see Webb as a president in 8 years in Obama goes the distance?
Obama/Rockefeller - Interesting pairing, and probably a better long-term look at a Obama 08/12 and Rockefeller 16/20. The guy can't be bought, I'll give him that, and he comes from a long historical line of aristocratic benefactors who believe "to whom much is given, much is expected", which actually isn't that unappealing.
McCain/Crist - I don't see this happening, and from living in Florida I'll tell you why. Crist is a perennial pol, and for all that people like him down there, he's fought "gay" rumors for years trying to get elected to this, that, and the other office down there (usually failing). He might be arrogant enough to think he should be vice-president, but I don't see him surviving the spotlight without blowing up into a drag on the ticket. Given their reliance on the anti-gay vote, I don't see Repubs accepting Crist on the ticket.