A lot of bloggers have worried about remarks made by "anonymous" commenters on their posts. The cloak of anonymity does indeed seem to be the refuge of the extremist and that's commonly where you'll find most offensive remarks, some of which may seem libelous.
Britain's High Court had to wade into this very issue in a case involving the often controversial paper, The Daily Mail. The long and short - a woman complained to a municipal authority about some bum in a park; the municipal authorities found the woman's complaint threatening and added her name to a list of potentially violent people; she sued the authorities and won damages for libel. The Daily Mail carried a report on the case on its web site and that attracted a couple of "anonymous" comments that the woman found defamatory. She then sued the paper demanding it disclose the identities of the commenters so she could then sue them.
Madam Justice Sharp dismissed the claim.
"It was fanciful to suggest that a sensible and reasonable reader would understand those comments as being anything more than 'pub talk'. ...The postings were of two lines and were effectively posted anonymously by members of the public who did not report to have knowledge of the matters they concerned. It is important to put the postings into context as to their meaning and what they were commenting on."
Now this is, of course, a British case and the court there was following, in part, the European Convention on Human Rights. A Canadian judge might see the same facts differently but the specific finding that most anonymous comments are so much "pub talk" is certainly helpful.