There's a very interesting post over on PHPComplete today concerning an annoyance that has been showing its ugly head quite a bit lately - comment spam - and how it relates to a website's Google ranking.

If you're a blogger (or a blog reader), you're painfully familiar with people who try to raise their own websites' search engine rankings by submitting linked blog comments like "Visit my discount pharmaceuticals site." This is called comment spam, we don't like it either, and we've been testing a new tag that blocks it. From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in our search results. This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.

You'd still have to go back and update the anchor tags to get this to work (which kind of defeats the purpose of making it easier on the site admin), but it is a really good idea...

See also other people's opinions on the matter - Jason E. Sweat, Derick Rethans.