Google, Yahoo, MSN, MovableType and others have agreed to not boost search rankings to sites with links marked with the rel="nofollow" attribute set. While this doesn't physically prevent comments on blogs from being created with links back to sites, it does prevent them from doing any good on search rankings. I'm very impressed to see these major search providers working together, as well as the major blogging software packages all jumping on this standard. An elegant solution that everyone can get on board with.
I've updated the site to mark all links within comments with this attribute automatically, so only links within the main posts will do anything to boost search rankings of sites. I believe this also closes the door to trackback links boosting the value of various blogs.