Is it possible that the McCain campaign (and more likely, the RNC) chose Palin as a means to save the Alaska senate seat for the Republicans?
I just saw Chuck Todd and John Harwood on MSNBC saying that the McCain campaign is touting a post-Palin bump in the polls in Alaska. They wondered why the campaign would have spent money on a poll in a state as solidly red as Alaska. But it occurs to me that even if the presidential race isn't exactly tight there (and the polls have been tighter than anyone would have guessed), putting Palin on the ticket would get Alaskan Republicans out to the polls, and potentially create some coat-tails for Ted Stevens.
Even if McCain loses, this could prevent the Democrat Mark Begich from capturing what before Senator Stevens's indictment, was considered a long shot victory in the Alaska senate race. I guess it makes some sense for the NRSC to stem the bleeding wherever possible.