I believe there was a case some years back involving derogatory comments someone made (anonymously, they thought) using AOL's message boards, back when AOL was a large force on the Internet. It involved a guy who was a stockbroker or businessman or something, and he had money to pursue the case.

I don't remember details, but I think the libeled party sued AOL, did discovery to get the info on the "anonymous" poster, then named both AOL and the poster. I think I remember that the DMCA "safe harbor" for ISPs either came about because of that, or else it was the first test of the "safe harbor" provision of the law.

Perhaps someone else remembers more details.