If McCain loses in November there's no guarantee that Sarah Palin will be a front-runner to secure the GOP nomination in 2012 (though I'd guess she will be a contender). But if McCain wins and, as seems possible, serves just one term then, clearly she would be. Equally, if Obama loses this year there's no certainty that Hillary Clinton will automatically be the prohibitive favourite for the Democratic nomination in 2012 (though that too seems quite possible).
But, one way or another, the combination of Hillary's near-miss this year and Palin's presence on the GOP ticket makes me think that it is more likely than not that a woman, even if not necessarily one of these two, will be the Presidential nominee for either the GOP or the Democrats in four years time.
With women having come so close this year (whether on merit or not, and good people can disagree on this) I think we will find that there will be increased pressure to put a woman in the top slot next time. One more heave and all that. The momentum is with the ladies. And since married women are a vital constituency I suspect both parties will not want to be left behind in this race. In fact, it's not impossible, one way or another, for the 2012 election to be between two female candidates. That may still seem a stretch, but it is not, as I say, beyond the bounds of comprehension to anything like the extent it might have been just four years ago.