This is more than just Palin, although I'm sure she was the tipping point. This whole effort to get Hillary supporters has turned out to be a waste of time, because there aren't many of them who haven't made up their minds already on what they're going to do. It's entirely possible that many of the women who switched to McCain now feel so pandered to and manipulated have just decided to sit the election out. That movement is an underrated aspect of the recent seperation in the polls, women aren't a monolithic voting block and apparently are resentful of those who treat them like one.
What Palin has done is energized the Republican base into a bit of frenzy. But all that does is get people who were going to vote for McCain anyways want to vote for him more, which really isn't the point of a campaign when it's all said and done.