This case had to do with AOL's chatrooms, but some of the legal issues seem to me to apply to message boards as well. Essentially, AOL could not be held liable for the actions of a third party using their service to "defame" another chatter.

Here's a link to the case summary:

http://legal.web.aol.com/decisions/dldefam/green.html

Rosemary O'Neill
Co-Founder/President, Infopop Corporation