Since as far back as I can remember, Slashdot has allowed people to post anonymous comments on any entry. The catch is that they had their posts automatically knocked down a level and they were referred to as "Anonymous Coward". This worked pretty well to keep trolls less visible without having to spend a lot of moderation effort on it.
In the brave new world of blogs, we don't have similar protections against anonymous cowards like this. Trolls and spammers can pop in, leave a nasty comment, and be just as visible as the next person. Thankfully, we have plenty of other tools at our disposal: Akismet spam filtering, site-wide moderation, and user registration. Since we can interdict these comments before they are committed to the public, we now have to figure out what to do with them. Personally, I approve all comments on this site that aren't spam and let them speak for themselves. (I'd probably filter out some profanity, but I haven't had to do that yet.)
I've seen many nasty comments left on blogs (particularly political ones) by those who choose to remain anonymous. No name, no e-mail address, no blog site, not even a pseudonym. It bothers me that someone can think "hey, I'm going to be a jerk and suffer none of the repercussions!" I have a rule for myself: accountability for what I say and what I write. I don't hide behind a pseudonym, I always provide a way that I can be contacted, and I don't set out on a mission to stir the pot.
What amazes me about these people is that they wouldn't act that way if you were having the conversation face-to-face. Add in a pinch of anonymity or even just the distance of the Internet and you get the kind of bad behavior that even children know isn't okay. It's kind of the same as road rage.
I think about all that can be done about anonymous cowards is to let them do their thing and show the world how pathetic they are. They'll get the message sooner or later.