This is pure speculation, but if I had to guess, I'd predict Sarah Palin will not run for President in 2012. For personal reasons. Let's face it--a person with 5 kids, including a special-needs child, can take off two months of her life and run for Vice-President. And let's further face it--Vice-President is not that hard of a job. But taking off two years away from home to trudge around Iowa and New Hampshire? Honestly, I don't know why anyone would want to do that--I read that Chris Dodd actually enrolled his kids in Iowa schools when he was running for President. That's pretty weird, if you ask me. You have to be relatively unusual person (ok, "sick" is the word I'm really thinking of) to undertake the project of running for President.
As for Palin, of course I don't know that much about her or what makes her tick. But for all her quirks and controversy, she seems like a basically sane, balanced person and a dedicated and fulfilled mother. And she and her family appear to really like living in Alaska (Tina Fey's jokes beside the point). So it seems to me that going through this maelstrom for 2 months as a Vice-Presidential candidate traveling with her family in the high style of a national presidential campaign is one thing. But for two years on the rubber-chicken circuit? It is hard for me to believe that she'd put herself and her family through that. But that's just a guess.
If McCain had won, that might be a different story. She could presumably could have balanced a campaign for the presidency with her duties as VP. But I don't see it happening otherwise. Although perhaps when her kids are older, especially if she ends up in the Senate at some point.
Since it is never too early to start the next campaign, I'm guessing Romney's the Republican front-runner for 2012 at this point barring some unforeseen new face on the scene. My sense is that a lot of Republicans already had buyers-remorse that they didn't rally behind him this time, especially once the economy emerged as the major issue. And Eugene's post on the Georgia run-off suggests that Romney is already committed to traveling the country building up support between now and then. Republicans are going to get used to him and trust him more between now and then. Talk radio and grassroots conservatives warmed to him a lot when he remained as the conservative alternative to McCain. Finally, unlike Guiliani and Thompson, Republicans seemed to become more attracted to Romney the more they saw of him. Huckabee will probably be around, but I can't see him as being more than a spoiler again.
Who might be a possible new face on the scene? The most likely candidate, I think, would be Jeb Bush. He's generally regarded as the more able of the two brothers anyway. And one could imagine him have the organization and fund-raising potential to emerge four years from now. And, obviously he is from an important state and has the potential to recapture some Hispanic support for the Republicans. And every conservative intellectual I've met who has met Jeb has really been impressed by the guy's smarts and commitment to conservative ideas--much more than his brother and his father. The drawbacks of being another Bush are pretty obvious as well.
And as a courtesy, I'll even start the Comment thread. "Jeez, we just finished the last election--do we have to start talking about the next one already?" My defense--I'm blaming it on Eugune for his 2010 post!