Dan, I think that the Michigan law is keeping Romney from making a huge mistake. He's already raised a lot of right-thinking conservatives' ire for his apparent inability to maintain his position on anything when it is not expedient to do so, and for his apparent lack of a core or compass strong enough to outweigh his personal ambition.
Leaving Massachusetts (and the beliefs he professed while there) to run for President as a newly-minted "conservative" contributed to that perception greatly. Picking up stakes again and moving to the next state that might vote him into a position of power would reinforce that image irreparably.
I'm not sure exactly where Mitt needs to go from here, but moving to another state and running for another office probably isn't it.