From Google Blog:
If you’re a blogger (or a blog reader), you’re painfully familiar with people who try to raise their own websites’ search engine rankings by submitting linked blog comments like “Visit my discount pharmaceuticals site.” This is called comment spam, we don’t like it either, and we’ve been testing a new tag that blocks it. From now on, when Google sees the attribute (rel=”nofollow”) on hyperlinks, those links won’t get any credit when we rank websites in our search results. This isn’t a negative vote for the site where the comment was posted; it’s just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.
What’s really nice about this current effort is that it involves a partnership between Google, MSN Search and Yahoo! on the search side and several popular blogging sites and software including WordPress, Livejournal, Blogger and MSN Spaces among others.
The linked article has information on how you can get involved with fighting comment spam through your blog today.