Blog spam.
Discussion on how to block blog spam is going on over at Feedster.
Here's one thing I found remarkable about the comment spam I got so far: Every single notification e-mail MT sent me about such comments was caught by spamassassin. The best way to attack this is probably by not reinventing the wheel, but marrying e-mail anti-spam tools with blogging software. Could be as easy as turning a comment into a fake e-mail message and handing that off to spamd before you accept a comment.
Also, the blog world might wish have a look at some of the other lessons learned by the e-mail antispamming community. One of these: When there is a central point of failure that can make many spam filters fail at the same time (like a block list), then that service is attacked until it's unavailable.
