Has anyone hear ever heard of such a thing in a lit contract? I sure haven't, and I certainly wouldn't sign any contract which had one.
From Absolute Write (post #89 and a bit of commentary after) through EREC, someone named Cant was asking whether there's a way out of a contract if you strongly suspect your publisher is about to take a nose-dive, but he didn't want to give many specifics because there was a non-disparagement clause in his contract. (It came out later, though, that it was Cacoethes Publishing, and the discussion was moved to that thread.)
Beth Bernobich, in post #96, said she had a non-disparagement clause in a short story contract once, and that the (unnamed) publisher refused to budge on it. That shows that it's not just a fluke with a single publisher.
In all seriousness, the presence of a non-disparagement clause or anything similar in a contract is, to me, a huge sign lit up in neon saying "We're going to mess you over and don't want you complaining to anyone about it."
I guess that makes it a good thing -- nice of them to give writers such an obvious "Flee For Your Life!" signal. [wry smile]
And just what would "non-disparagement" cover, anyway? Would it cover anything negative or even questioning one might say about the publisher? That sounds awfully open-ended; I didn't think the courts liked that sort of thing. Or would it be more like existing libel and slander laws, where you're free to say what you want so long as you can show that it's true? If that's the case, then a non-disparagement clause wouldn't prevent someone from posting on their blog that "Fly By Night Press hasn't paid me in three years, hasn't answered their phone or e-mail in two and a half years, and I just spotted my book being sold in B&N with the FBN owner's name on the cover." (Assuming it was all true, of course.) So what's the point, then?
I'd love to hear an opinion from someone who's actually up on related law, because this sounds incredibly iffy.