Certainly the big three of South Carolina, Florida and Georgia, probably Alabama, Mississippi and Tennessee, NC and VA likely tossups.
Romney is a Yankee, even if he is supported by the Texas oil establishment - the Mormon issue is a factor, but regionalism trumps religion here. Romney takes most of the Northeast, Texas, California. Gingrich takes the Rockies except for Utah, and the Midwest gets split up between Romney and Gingrich, with Santorum taking Indiana. Ron Paul slowly accumulates a lot of 2nd and 3rd place wins, not enough to gain the nomination, but enough to be able to be a power broker.
Way I see it, no one comes into the convention with a clear majority, perhaps not even a plurality. In the end, I still think it will be Romney with the largest absolute vote count - outside of the deep south most Republicans despise Gingrich nearly as much as they do Obama - but there will be a lot of horse trading at the convention itself.
All of which is probably good news for Obama. He's actually managing at this stage to be positioning himself as the adult in this election (which is even funnier given that he's younger than all but Santorum), and while support for Obama will likely be tepid compared to 2008, too many democrats are energized to come to the polls for other reasons this year.