Google Inc., Microsoft Corp.'s MSN division, Yahoo Inc. and Six Apart Ltd. announced late Tuesday that they are supporting a tag called "nofollow" to exclude links in blog comments from search-engine crawlers and to prevent spam posts from influencing search rankings.More info at eWeek
Major search engines to support "nofollow" HTML attribute to fight comment spam

Google, MSN, Six Apart and Yahoo announced to support a new HTML attribute in their search engine algorithm to fight spam in comment sections of blogs and other websites. If webmasters use this then it will prevent that links posted by users get spidered by search engines.