Mitt Romney is courting the vote of conservatives. If he becomes President, I wouldn't expect him to stab in the back the conservatives he's going to need in 2012.
I'll admit it's certainly possible he could abandon conservatives once elected, figuring that that name recognition would allow him to win in 2012 even without their votes, but the risk of that happening is less worrisome than the near certainty that a candidate who is more interested in chasing liberals' votes rather than earning conservatives' votes will run even further left once elected.
I know McCain is unacceptably bad (McCain-Feingold, among other things). I know that Rudy is unacceptably bad (gun lawsuits; sanctuary city policies). Of the people who would seem to have any chance at all, Romney is the only one who is even marginally acceptable, and he seems like he might actually be fairly decent.