I'm just on my way back from the APSA conference. Needless to say, Sarah Palin was all anyone was talking about between panels, and often during them. I talked with a lot of political scientists about it. There is no consensus on whether this was a smart or dumb move by McCain.
Probably the modal reaction was that McCain looked as his internal polling numbers and figured he was screwed, so he needed a game-changing roll of the dice. Sort of like Mondale's decisions to pick Geraldine Ferraro and to promise to raise taxes. In the Palin case, maybe it helps McCain pick off a few disaffected female voters who otherwise liked Hillary and really, really don't care about policy. If not, he still loses, but the GOP looks more impressive in the long run for the effort.
Personally, I think this was kinda stupid. Not that she's a bad candidate (although it's hard to imagine she's ready for the media exposure she's now getting). She may be very good. The main problem is that she undercuts McCain's main rationale for running against Obama: experience. That was probably McCain's strongest issue. Now the two tickets probably average about the same level of experience.