Interesting article on the use (and need for) rel=nofollow links to prevent link spamming. Google appears to be penalizing sites that have what they perceive to be as link spam. One method to remove that risk is to have all user generated links be marked as rel=nofollow. That way, google won't use them in generating PageRank for other web sites.

The downside is that Google is missing out on a source of content quality - links created only by the motivation in identifying good web sites. If all user-generated content is deprecated, then we only have the "web site publishers" who are ranking the pages on the web. That seems lop-sided and wrong.

The problem is - when should a user be trusted to not abuse the link generating system. This article discusses various Karma-scoring systems that preferentially remove the nofollow attribute from trusted users.