It’s interesting that the story on bloggers and defamation has so many comments today. One blogger told of his experience when sued for defamation
Having had a defamation complaint about a comment made on my site there’s good news and bad news.
If you delete it and are willing to hand over the log details of the commenter then you’re going to be very hard to pursue.
If on the other hand you’re not willing to do that (and there are good reasons not to in some circumstances) then you’ve got a problem.
But if some unknown third party is using your blog to launch unfounded attacks on others then why should that be your problem?