If McCain wins, Obama loses. You don't get to miss an opportunity like this and live to try again, so Obama is out for the next one too. The Democrat next in line will naturally fall to Clinton, as the primary runner-up and scion of the Clinton family. With her and her husband's speech last week, they plausibly claim they made the effort and wash their hands of the electoral loss, and can instead argue that blame lies with Obama's failure to use Mark Penn-style hardball tactics against the Republican smear-mobile.
Judging a contest between Palin and Clinton four to eight years from now is a stretch, but I'm going to call it for Clinton anyway. Republican fatigue and lingering Democrat anger should help her (though demographic reshaping after the 2010 census may help Republicans). Again, when have there ever been three consecutive elected heads of state from a single party serving full terms?
So by voting for McCain now, you help move Bill Clinton into the White House again. Something to keep in mind this November.