I see that Timothy Sandefur has posted a piece on libel law and Mims.
I wouldn't be surprised if Pianka's another story in this regard though; I mean, a) Mims (and Dembski?) either implied that Pianka advocated a holocaust or he didn't, and b) Pianka did get death threats.
I'd submit that if point "a)" is patently, and obviously false, and "b)" indicates damages, Pianka's case against Mims is pretty strong.
Of course, I'm not a lawyer and don't play one on TV