The Pelini gone rumors have been around for quite some time. I wouldn't be surprised if this was his last season. He's alienated the fan base and Husker fans (albeit unrealistically) expect more than what he's done. I have some friends that are huge NDSU fans, and they're quite nervous that Bohl will be gone after this season, especially if the Bison win another FCS title - he has nothing more to prove, and his window for making big time money might be small. He'd be an obvious fit for NU - he coached there before, and if he likes the culture in Fargo (which he does apparently), Lincoln is pretty similar).
Fitz is a lifer at Northwestern. He's done more there than anyone could ever expect, despite this unlucky season. It's easy to compare NW to Stanford, but the bay area is a much easier place to recruit than Evanston for those that qualify.
Petersen is an interesting case. Supposedly he stayed at Boise because he has a son with some medical problems, and the family has been really happy with the care he's had there. Not sure what the situation is now, but there have been rumblings that he's at the point now where he would listen to offers. The most obvious at this point would be USC, but culturally the difference between LA and Boise would be like Earth to Mars. Would he make that jump?
Nobody is going to hire Tressel right now.