If Senator Clinton wins the Democratic primary (and let's remember that it is she who is running, not Bill, Release 3.1), why would Obama run with her? It would ruin him. She would never allow him to play a meaningful role in her government, and he'd be slowly filled with the special poisons that seem to be dripped into people who spend an extended period in Washington.
If Senator McCain wins the Republican primary (I think Romney is the more likely winner), why would he want any of the third-stringers, vacuum-brains or bean-counters as a running mate? He has nothing to lose by picking up the phone and calling Obama.
Given that everyone would expect McCain to be a one-termer, thousands of independents and maybe 5-10 percent of Ds would defect. I think McCain/Obama would run above 55 percent in a general election.
Even better, that kind of ticket would not have much effect on House or Senate races, except to encourage the extraction of time-serving nitwits of both parties.
I'd rather see Obama as the Democratic nominee and McCain as the Republican, which would be good for the country and pretty much ensure a serious hosing-out of Washington. But if we can't have that race, let's see them team up.