I think McCain made a canny choice. It was a bold one, anyway: this move is either going to look nuts or brilliant in under a month.
Depending on the polls you believe, between 20% and 30% of Hillary Clinton voters were self-reporting as "likely to vote for McCain," and that's *before* JMC picked a woman as running mate. So there's the obvious, transparent and cynical appeal to disgruntled Clintonistas. But the thing is, this might work!
Independents are still very much in play. McCain himself, though pro-life (as is Palin) can best be described as a moderate social conservative with occasional tendencies towards actual tolerance, and Palin has been described as "as libertarian as you can get and still be on a major ticket" by no less than the editor of Reason magazine.
Plenty of people have modestly libertarian views (including many independent voters!) - economically conservative but socially tolerant.
Best punditry I've seen so far: