Official Google Webmaster Central Blog: Hard facts about comment spam
Would seem that google is about to drop the hammer on blog commenting for backlinks. I say "about to" because despite what Google says they are already doing I know of quite a few sites that are ranked well on blog commenting.
The article actually directs webmasters to go back and remove them as if theres some benefit to removing the links . Don't do blog commenting for backlinks myself but I know a few people who really depend on it. still a part of me believes this is just blowing smoke because the whole point of the nofollow tag is to leave it up to admins - but we'll see.