All right, my take then:
Obama will be the President and incumbent in 2012. For any Republican to have a decent chance, he will have to have really screwed up by then, and have the blame attached to him. The best opportunity is if unemployment is still high, which sucks for the country but is good for opposition.
Obama ran as a fresh-faced "change the tone in Washington" populist. If he ends up in 2012 as being vulnerable, I don't think Palin's chances are good as being the populist replacement. I think the country would be looking for more stability. Old guard...probably.
Don't think it will be Newt or Romney, though.