Sarah Palin's nomination for VP, while temporarily energizing McCain's campaign, will ultimately destroy any chance he might have had to win this election. The damage to his chances has nothing to do with Trooper-gate, her pregnant daughter, her views on abortion/creationism/etc., her questionable record on the Bridge to Nowhere, or harbor seal DNA (my first self-link. Big moment!). It has to do with Barack Obama.
What was the one real argument the Republicans had against Barack Obama? It wasn't Jeremiah Wright or Bill Ayers. In the current anti-Republican environment, it wasn't the issues. The "He's a Muslim" angle doesn't work outside of whack-jobs. It was experience. I'm as big an Obama supporter as you're likely to find anywhere, but he is, compared to your typical Presidential candidate, rather short on experience. We can argue all day about whether that deficit matters, but it's undoubtedly there.
Sarah Palin's nomination takes this line of attack and turns it into so much moose stew. A candidate's VP choice is the Presidential equivalent of choosing a godfather for one's children. "If something happens to me, here's the one person I think is best to take care of my country/child." And McCain chose Palin, who at best has as much experience as Obama. So the next time any Republican attacks Obama's resume, he only has to point to the woman Republicans seem to think is the 2nd most qualified person in the nation to lead it.