In libel cases specifically, if you can prove who the author was, you can sue. If you can't, too bad. Heh, heh, heh.
Be careful here - this does not allow third parties a blanket escape from liability simply by disclaiming authorship. As I understand the law of defamation, who *wrote* the words is not all important; the issue is who is publishing or otherwise causing them to be published (which would ordinarily include, but not be limited to, their author). For example, in NY Times v. Sullivan (the case that established a different standard of defamation for public figures), the NY Times didn't write a story, they simply carried an ad that claimed abuse of power by some officials (in Georgia, I think). Although it is often easier to show that the author of defamatory words knew or should have known them to be false (part of what you need to prove to win a libel case), liability does not end there. Knowingly repeating words you should know to be defamatory is still defamation.
I asked Mike Godwin about this specifically a few months ago. I mention him here to give him to opportunity to correct or elaborate.
Eric
-matt