While a newspaper has a responsibility to check the accuracy of letters to the editor, if person A were to start a cafe, and person B walked into the cafe and made statements that the court deemed libelous, it doesn't seem likely that person A should be held responsible.

If Person A rents a hall, calls a public town meeting, and invites people to walk up to a microphone and say whatever they like, and Person B makes statements that a court deems libelous, would person A be legally responsible for any offense committed by person B?

I'd welcome opinions, but links and evidence would be even more welcome! More on weblogs and libel over at my blog.