Let us examine several data points, which I freely admit are merely put out there to mess with various heads. There's no valid reason to take any of these seriously.
No, really. Not at all. I'm just doing it to be cruel.
First. Senator Barack Obama never had any intention of picking Senator Clinton for his running mate; no vetting process ever took place.
Second. Governor Sarah Palin has been on Senator John McCain's radar for a Vice Presidential pick since February of this year: right wing pundits and activists in general have been talking about her for a couple of years now. (Via Hot Air, and watch those videos to see why.)
Third. I see Ed Rollins' point. If Senator Clinton was going to be the VP nominee, Senator McCain would have problems picking Governor Palin as his running mate. (I also agree that Senator Obama made a drastic mistake here by not picking Senator Clinton, but never mind that now.)
Fourth. Senator McCain's determination (as per the WaPo) to keep Governor Palin in the running for the Republican VP slot must be examined in this light; up until quite recently, the illusion that Senator Clinton's name was in consideration for the Democratic VP slot was quite real, as it was easily the most sensible choice from both an internal and external viewpoint.
Fifth. To recap: Obama and Clinton knew all along that Clinton would not be considered for VP; McCain's growing determination to have Palin for VP is thus a bit of a risk, given that he never had the chance to look at the Democrats' cards...
Sixth. Senator Clinton and Senator McCain are actually close personal friends.
Enjoy your night.