I posted an article over on the WordPress development blog earlier on the subject of blog comment spam.
Why do spammers post comments on blogs? Do they really expect blog owners to leave their comments up for people to read? Do they expect people to follow their links to the spammers’ sites selling viagra and porn? Not really. What they are really after is search engine ranking. Because the more links to their sites that a search engine sees, the better the chance that their site will come up in a list of search results. Even if you’ve deleted the original comment, it won’t matter, if a search engine has already indexed the links in the comment before you deleted it. So, one of our anti-spam tactics should be to bust up the search engine ranking for spammers links.
I am also going to be contributing to the blam! project, which is an effort to create an open distributed method of maintaining a network of spam blacklist information.