Matt O'Neill has done a post on his blog asking the legal position on external comments on one's blog.
He refers to an e-mail/telephone discussion which we had as well as a discussion with Michael Dillon of Sun Microsystems.
It is worth a look because it shows the tough call that this issue is. My own reason for having comment moderation(and this is raised by Michael Dillon) is not primarily legal but more that I do not want any unnecessary bad-mouthing of my site. I think it is a branding issue rather than a legal issue.
That said as we all know defamation laws are much stricter in the UK than the United States and as the recent instance of baby guru, Gina Ford and Mumsnet shows this is a real issue. Also a very good article has been written by John Naughton.