There are several possible choices. A couple of people have brought up Lieutenant Governor Risch. I think it'd be a hoot if he named Robert Vasquez, myself. He's the guy who sued Mexico on behalf of Idaho taxpayers. The illegal immigrant activists' heads would explode. Ain't a-gonna happen, though.
Craig may simply decide to ride it out - man, I have to be careful with the phraseology - and not run in '08. Field is pretty much open anyway. That'd make the next session in DC pretty interesting, and I don't think it'd do the Republican party a lot of good if he did so. My guess is that he'll quietly resign.