First let me say thanks again for your sponsoring part of my trip to the MySql conference! I really appreciated it.
I've seen this nofollow post before and considered doing it but never got around to it yet. I'm getting virtually no comment spam using ReCaptcha so I guess it hasn't come up as an important priority. I'm considering implementing it though. Currently we use NeatHtml for prevention of cross site scripting in the comments so I sent Dean Brettle an email to see if he might be interested in adding this as a feature in NeatHtml.
The only thing I'm unclear about is do I really want this for all links entered in the comments? I mean if real people like you post a link in my blog comments why not just let it be a normal link? If you post a link to your site I don't mind it being noticed by the google crawler. If someone posts an inappropriate link I delete the comment. I get an email whenever comments are posted and I always check them to see if any spam got through.
So I'm just wondering if its going to penalize good people who I don't mind linking to.