Danny Sullivan is on a mission to do something about link spam (comment, guest book, trackback, forum, and so on). In a recent entry at the SEW Blog, Danny wrote on the topic of Google To Add "Nofollow" Tagging Of Links To Fight Spam? The proposal is to use the following within your links; <a href="http://www.site.com/page.html" rel="nofollow">Visit My Page</a>
By adding rel="nofollow" it would instruct the search engines to not string along any "vote" "weight" "importance factor" to the page it is linking to.
Forum discussion at Search Engine Watch.