Chuq’s comment make sense, but Brad takes the next step: I think Google would do well to try that.
Chuq Van Rospach on Comment Spam:
If I were google, I would have taken a different approach. I would not have said, “if you make sure links have a ‘nofollow’ tag we won’t include them in our index.” Instead, I would have said, “Links that are created by somebody other than the page author — i.e., links inside blocks labeled as comments — will *not* be included in our index unless they have a ‘follow’ tag inside them.” Given where we are now, I think the second approach would have done more good and also improved the quality of google’s indexes.