Rogers Cadenhead has come up with a rather innovating anti-spam technique:

I'm trying a new technique this week that makes spam easy to detect by putting a bunch of bogus text areas on a weblog form, hiding them with Cascading Style Sheets, and checking them for input when the comment is submitted. I call these fields comment flak .

Spammers typically put their junk comment in every text area on a form. When text shows up in any of these flak fields, my blogging software treats it as spam.

That's a brilliant idea. I'll have to look into that.