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

Basically, you get these people who post thier links in your site in hopes of boosting thier own page rank on the search engines. Google has identified a need to block these links from getting credit in the search engines and has setup a tag that thier engine will use to ignore such links.

The blog community is worst hit on this so its started there but I would have a look at it and see if this isnt something that could be implemented into the IF forum as well.