Agree with what Jon Chait says here–if a Republican wins in 2012, we’re getting stimulus. The GOP will want the guy to get reelected after all.
Here are my premises. Most Republican embrace of contractionary fiscal policy reflects conscious or unconscious partisanship rather than a sudden conversion to Austrian economics. You do have some genuine crazies in the House, but Republicans like Mitch McConnell are mainly driven by a desire to win power and please business. So, if the 2012 elections flip the House to the Democrats and put a Republican in the White House, that president will quickly grasp the policy and political imperative of stimulating the economy. The Democratic House would support it for policy reasons, and the GOP senate would support it for political reasons.
Chait’s scenario is missing one important part: The moment when Republicans attack Democrats for not supporting the conservative stimulus they’re proposing because they want President so-and-so to lose. Not that they’d do anything like that.