The upside for Romney is he doesn't have to be governor during 2G2D. None of the current governors will be able to run on records of balanced budgets, tax cuts, and sufficient public services.
Palin's an idiot, but so was Reagan. Given the right support and prep work, why not? The question is whether the people around her are too amateur-hour to build a network over the next three years.
If Sanford runs, it could help a moderate-ish candidate, counterintuitively. Everyone else could write off SC as a favorite son primary and skip it, making it less important to appeal to the Left Behind wing.