Well, of course they do. They did so in email, too, and now I get hundreds of spams a day. Doing it on the web is even more insidious, as recent wiki spamming indicates. This guy (calling himself a "search engine optimizer") inserts links to his site into wiki pages. Why? It inflates his Google pagerank.
It's just a matter of time before someone starts crawling the web looking for open comment/trackback forms and doing the same thing. (Also, this guy reportedly did it manually; a script would be much more effective.) What do we do then? Me, I'll just laugh at them from the safety of LiveJournal. On the Atom list, they're talking about how to combat this manually(!?) by creating notifications that let them be notified of the spam so they can revert the pages.
(Though my email situation still sucks. And if LJ ever drops invite codes, the problem returns...)