I'm at a panel discussion on community at Blogher and the issue of having a comment policy has come up. I did a bunch of research on this topic, so here's a summary: After reviewing a ton of blog policies, I discovered three basic types of comment policy:

1. Free speech zone.  No holds barred, anything goes (except spam).
2. Discourse zone.  Attack ideas, not people.
3. Living room zone.  I'll let you say things in the comment section that I would find acceptable if you said them to me face to face in my living room. 

You can browse examples of lots of different comment policies here: del.icio.us/blog_policies

Blog policies are a lot more than comment policies.  Take a look at We're Making the Rules Around Here:  Blogger Created Blog Policies, or listen to audio of a discussion at the Berkman Center for the Internet and Society on the rules bloggers create for their weblogs.