From Google blog:
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.
So what the heck does that mean?
For example, I am always getting spammed with comments for the diet pill Phentermine. So a spam comment might look like:
<a href=”http://buy-phentermine-online.com”>Online pharmacy Phentermine.</a>
The spammer’s hope is that if enough of these comments get through on a couple dozen blogs, their search engine ranking will increase.
What blog tool programmers are going to do is have the software that publishes the blog to ‘tag’ URLs in comments with what’s called an attribute.
<a rel=”nofollow” href=”http://buy-phentermine-online.com”>Online pharmacy: Phentermine.</a>
The attribute ‘nofollow’ is sort of like tagging a link to tell search engines Don’t follow this link – I didn’t put this here, a commenter did. Of course you as the blogger can still always edit, revise and remove comments as you see fit.
They are ‘de-weighting’ links in comments so they won’t affect the commenter’s ranking (this shouldn’t affect the blogger’s ranking).
Developers of LiveJournal, MovableType/TypePad, Blogger, WordPress, Flickr, Blojsom and Blosxom have joined in. The article also says that Yahoo! and MSN consulted so look for their search engines to join in too. It’ll probably take a few months (years?) for everyone to upgrade and for the net effect to be enough to shut-down blog spammers.
So what the heck do you do now?
Blogger users. Nothing – they’ll upgrade automatically.
TypePad users: Nothing – they’ll upgrade automatically.
MovableType users: Plugin.
Full details: Preventing Comment Spam