The idea that a district attorney cannot be sued is one of those theories which, in real life, can only be stretched so far. Much of what I read indicates that not only is Mike Nifong way beyond the breaking point on that one, but that the city of Durham NC itself and Duke University itself are facing major kinds of legal liabilities in the lacrosse case.
An article on Lew Rockwell's site spells out the liabilities which president Brodhead and other members of the administration and faculty of Duke face:
http://www.lewrockwell.com/anderson/anderson164.html
As that article notes, Duke has legal liabilities which might be big enough to shut the place down. They were contractually obligated to protect those kids on the lax team, and not to aid and abet black panthers talking about beating confessions out of them, people posting wanted posters with their pictures over the campus, chanting "CASTRATE" at them, or teachers calling them rapists in their classrooms.
At the least, I would anticipate a massive class action lawsuit as soon as the Nifong "rape" case is shut down.
It also seems at least theoretically possible that the administration of Duke and some of its faculty could be charged with conspiracy to commit assault. Assault is usually construed to mean simply placing another person into a situation in which he could reasonably expect to get hurt or be harmed.
An idea of the liabilities which Nifong and the city/county of Durham NC face may be had from an article on the Lincoln Tribune:
http://www.lincolntribune.com/modules/news/article.php?storyid=5701
John Banzhaf is with the GW Univ. Law Center and, as he notes, if somebody has the means and the will to go after a psycho DA like the Fong, and the families of the three accused lax players do, there are ways to do it. Fong has stepped way outside of the bounds within which DAs are protected from lawsuits. The county of Durham and the people who live there and who voted to return Nifong to office of course enjoy no such legal protections, and are fair game.
Generally, the one crime you can commit in America which is ALWAYS severely punished, is to embarass the power structure, and Fong has done that; he has made the entire legal profession in NC look like a third-world clown act.
Nobody should think he is going to get away with that.