This is just a new building block that lets software makers have better
granularity--at a link level instead of a page level. And they can declare
things about the authorship of a link that they couldn't before. Lots of people
are using it to do smart things; for example, LiveJournal can recognize
authenticated users and they won't get the nofollow tag. So it's not the case
that every comment or trackback will be reduced in value. Last week, a software
maker didn't have that ability, but now they do.
In other words, if you trust
a user or like their comment, it won't be hard for them to get search juice from
commenting on your posts. I believe that adding this flexibility for software
makers will be a very helpful thing in the long term.

google guy has commented in battle media blog here, battellemedia.com/archives/001197.php#9161 about the new nofollow attribute which will be added soon to comment spam to prevent comment spamming,

Read more on google's effort to combat comment spam here,

http://www.google.com/googleblog/2005/01/preventing-comment-spam.html

                                Earthlink Netscape Netvouz RawSugar Shadows Sphinn StumbleUpon Yahoo MyWeb