Google has added a feature to they search algorithm that ignores links that have the rel="nofollow" tag in them. This is great because it let's google know what information is your and what information was just posted by somebody commenting. They have recommended the nofollow tag be used on all links that can be posted by untrusted users. MSN Search and Yahoo! are both supporting it. Most blogging applications have already incorporated it. I am going to add it to my system too. Here is the link to the original article on Google's blog.