Don't sue climate change denialists
The Sandusky comments seem to me to be obviously permissible, since the CEI and NR didn't accuse Mann of committing specific acts of pedophilia. They fall into the same category of hyperbolic rhetoric as things like "BusHitler" or "Obama is the antichrist." Of more concern is Mann's claim that it's libelious for CEI and NR to assert that his research was biased. Here, he is essentially asking the court to adjudicate a dispute over the quality of scientific work -- to hold that CEI and NR were knowingly calling his research bad when it was in fact good. It seems to me that the court system is in a poor position to do this, and it sets a dangerous precedent for high-profile scientific issues to be subjected to the legal system.
The courts are simply not well-equipped to examine scientific questions. Judges do not have scientific expertise. The court system's own methodological procedures are based heavily on tradition and ritual in contravention to what we know (from scientific research) to be good practice in determining the truth -- such as the heavy reliance on unreliable eyewitness testimony. Perhaps more importantly, a key virtue of scientific procedure is its openness. There is always room for debate and reconsideration within science. Scientific claims never come to one final end where they are declared to be finished fact, though well-supported propositions (such as anthropogenic climate change) do, over time, accumulate broad-based assent. The whole point of the court system, on the other hand, is to issue final rulings. A court must issue a yes or a no, a guilty or an innocent, at the end of its proceedings.
I can understand Mann's desire to take action against people who have ruthlessly attacked him for many years and hindered his ability to conduct his research, but I think that when it comes to allegations about the quality of scientific research, the courts ought to maintain a very high standard for what counts as libel.