Google, Yahoo, MSN, several major blog software services are supporting the implementation of a nofollow link attribute in an effort to quell comment spam. Of course this relies on the owner of the blog or web site to actually implement the tag properly or at all, in order for it to be effective. As far as blogs go, I would expect the 80/20 rule probably applies in that the smaller percentage of blogs recieve the majority of traffic. Those bloggers are most likely to be very savvy to the comment spam issue because of their visibility and involvment. They’re most likely to start using such a tag. But it will take a long time without mass acceptance which appears to be happening.
For more information on the nofollow link attribute and comment spam, here are links to selected posts:
SearchEngineWatch Blog – Google, Yahoo, MSN Unite On Support For Nofollow Attribute For Links
SEOBook – More on the Rel=NoFollow Tag
Search Engine Roundtable – Reaction to Wide Search Engine Acceptance for the Nofollow Attribute For Links