Mark Glaser in the Online Journalism Review:
Just what the heck is "comment spam" and how did it get so many prominent bloggers up in arms? Basically, spammers have been using blogs to help boost their standings in Google searches by posting massive numbers of comments that include links to their pornography sites, scams and get-rich-quick sites. If your site is linked by a top-ranked site or blog, then Google will often raise your site's ranking -- at least that's the thinking of spammers.
I think that Google (and other search engines) has been tweaking its algorithm to downgrade the Page Rank of blog comments, so the spammers are ultimately going to lose whatever benefit they get from these attacks.
That doesn't mean they're any less annoying to deal with in the meantime, of course. I've got some 4000 pieces of spam to catch up on, so I've been shutting down old comments. (Yes, I know about MTBlacklist and MTClose, and I'll get around to installing them one of these days.) Eventually I'll just leave the main page open and I can kill new spammers as they arrive.
Jerks.
Via J-Walk