Shockwave Rider doesn't particularly like rel="nofollow" and it's come up in various other blogs too. The open letter from the S9Y developers to Google on the subject is also interesting.
I'm not particularly enthusiastic about it either - simply because it's the wrong approach. You can't repair a broken system by telling curious people to look away. Comment spammers won't be deterred one bit by the whole thing.
I can only agree with Phil Ringnalda that rel="nofollow" is something like the monster disclaimer of people on the web. Ultimately just as strange as the link distancing that many have on their websites - if you're distancing yourself, why link at all? If you're generally distancing yourself from your commenters, why have a comment function in the first place?
In any case, I won't be using rel="nofollow" - at least not by means of a large bucket that pours over everything just because it's a comment or trackback. Comment spam is addressed differently. If necessary, by putting everything in a moderation queue that then has to be cleared of spam using appropriate means - the same techniques used for email spam apply here. That's a far more worthwhile field of activity.