"Courts have said that because revealing a speaker's identity could end up deterring people from speaking up, we are going to require some showing whether there is a cause," he said.

Ed Yohnka, spokesman for the American Civil Liberties Union of Illinois, said the case was troubling. "We think anonymous speech on the Internet is really critical and needs to be protected," Yohnka said. It has traditionally been one way in which people have chosen to express themselves on political and social issues, he said.

Yohnka warned against a growing tendency by corporations and individuals to use defamation claims as a way to get the courts to force ISPs to unmask anonymous online commentators. "Saying something is defamatory shouldn't be the trigger" for deciding when someone should be unmasked he said.

Corporations and public figures in particular need to show they have a prima facie case before they are allowed to seek the identity of an anonymous poster, Yohnka said.