Prior to his VP pick, most voters were probably aware that John McCain was old-ish. But I'd guess they weren't preoccupied with his age, certainly not to the point of obsessing over his running mate because he might die in office. Now that McCain has picked someone (partly) intended to add youth and vigor to his ticket, every voter in America knows the actuarial math associated with a 72-year-old male and is poring over the qualifications of his veep nominee.
Which is why I think this is such a disastrous pick. After the novelty wears off, the take-away from Palin is going to be that the presidential nominee most in need of a qualified understudy picked someone you simply can't imagine serving as president.
As for the conservative refrain that Obama is similarly unqualified, well, it's just not true. Four years on the Senate Foreign Relations committee isn't exactly a lifetime of foreign policy experience. But it is (literally) infinitely more than Palin can claim, which is zero. More to the point, more than half the country does see Obama as a viable commander-in-chief, according to polls. Given that elections aren't about actual qualifications, but perceptions of qualifications, that's really all you need to know.
The only way Palin ends up helping McCain is if Obama and Biden overdo the skepticism. Obama's "60 Minutes" interview, in which he referred to her as a "fine mother and up-and-coming public service," is, to put it mildly, not an encouraging start. In general, I think the less said about Palin, the better. Her limitations will become self-evident as the media mines her background. Piling on is just going to create a backlash.
Update: Some commenters have asked what's wrong with Obama's "60 Minutes" comment. It sounded a little patronizing to my ears. As a friend put it, it's hard to imagine Obama saying "he's a fine father" if Palin were a man. On the other hand, I guess Palin and the McCain were the ones who played up her motherhood, so maybe it's fair game.