Over at Plum Line, I have a post up talking about what happens next if Rick Santorum wins in Michigan.
That said...Intrade has Santorum at 56% and Romney at 44% to win Michigan, and if I were betting I'd be with Romney. Nate Silver's polling model gives Santorum a 77% chance of winning there, but it doesn't know that Romney has a enormous advantage in resources. It also, on the technical side, treats Santorum's surge as a plus going forward. But I suspect that's probably wrong; in the immediate aftermath of Santorum's big day last week a lot of people were hearing only good things about him, but that's not all that likely to continue. And there's another thing...Romney has been far more successful in the debates than has Santorum. he's especially been strong in "confrontation" debates -- when a candidate has newly surged and everyone expects a showdown between the new polling leader and Romney.
The other thing to note is that Romney is very likely to sweep Arizona's winner-take-all primary on February 28, and if it's at all close in Michigan the delegates are going to be split evenly (or close to that). So Romney's delegate lead will get larger unless something truly shocking happens.
I wouldn't be shocked by a Santorum win in Michigan. But if you asked me to guess right now who would win that state, I'd be betting on Romney.