A really interesting article about Blog Comment Spam. This is one person's attempt to really follow all clues to their sources. It doesn't end all that satisfactorily but it's a good read. Coincidentally I just opened up comments on this blog and got hit with 80 bits of spam (and 6 real comments.)
For some crazy reason the comment spammers attack a single post. In my case, it was "Hacking the Papal Election" which alone got hit with the 80 comments.