Forgive me if I'm not gleefully giggling right now. This makes a lot of sense for McCain:

(1) It won't undercut his critique of Obama as inexperienced. If the punditry takes this line of attack - which it will - it has a real potential of backfiring, as it will only reinforce the whole theme that two years of national experience does not prepare you to be Commander-in-Chief. That's not a theme Obama wants to emphasize. And in picking Biden, Obama has already picked a VP that (at least superficially) embodies many of his main critiques of his opponent. This weakens his ability to play the card on McCain.

(2) McCain is making a major play for undecided women (which is the ONLY way he can pull off an upset here), and this is a smart gamble. If nothing else, it will mean two more months of hearing about Obama's "women problem" and PUMAs, whether that has any relation to reality or not. Again, not the best narrative for Obama.

(3) Placates the base without the (well-hashed over) liabilities of the other major VP contenders. Of course Palin brings here own - corruption scandals, the prospect of a disasterous debate with Biden, etc. - but at least in the short-run it seems good.

(4) McCain now has the sexy-librarian-fetishists on lock. Key demographic.