Poynter Online has a useful article up on assessing the legal risks faced by sites that publish user comments. The piece, while focused on traditional media organizations, provides some useful guidelines for anyone who runs a site that includes user submitted material.
According to attorney Robb Harvey, who is interviewed for the article,
among the considerations to be taken into account [is] the need for the newsroom to impose robust "terms of service" on all posters. Posters should be informed that they are responsible for their own postings. The newsroom should consider advising readers that the newsroom does not control or monitor what third parties post, and that readers occasionally may find comments on the site to be offensive or possibly inaccurate. Readers should be informed that responsibility for the posting lies with the poster himself/herself and not with the newsroom or its affiliated sites.
It's a short, but worthwhile, read.