Ben Hammersley's Dangerous Precedent explains why the excitement over "noFollow" in the Blogosphere is misplaced:
This is the key point. If rel="nofollow" works, if it's applied universally, it will actually have the reverse effect. It actually gets less effective the more it is implemented. Why? Because the comment spamming sites are in competition with *each*other*, and not with any legitimate businesses. They're not so much trying to get the best pagerank for their term, as trying to get a better one than their rivals. That's a key distinction. If the playing field is levelled by rel="nofollow", then everyone involved will be forced to try all the harder to get their links out there. The blogosphere will be hit all the harder because of the need to maximise the gains. As there's no more effort in hitting 6 million blogs as there is in hitting 1 million, this really won't bother the spammers one bit. All it does is shift the problem from the high pagerank blogs we here might have, with rel="nofollow", custom sanitize settings, and mt-blacklist in full effect, all the way over to the less technically adept. And that is one enormous customer service problem heading towards Blogger, 6A and the rest.
That's about the size of it. You really have to keep in mind that spamming costs the spammer nothing - so these techniques don't really affect their behavior. See Phil Ringnalda for more along these lines.
Update: Ben Hammersly updated the post I linked to with this excellect observation - one that I certainly hadn't thought of:
Meanwhile, Scoble points out how it can be used in other ways, and undermines the second aspect of the attribute: as respecting rel="nofollow" will involve losing an enormous amount of implicit metadata, any tools that are interested in that will be forced to ignore it. Technorati will have to choose if it's a site that measures raw interconnectivity, or some curious High School metric of look-at-that-person-but-don't-pay-her-any-attention that the selective use of the rel="nofollow" attribute will produce. For many purposes, this would mean the results are totally debased and close to useless.