Has it ever occurred to anyone to develop an “MT-Whitelist” as a different approach to comment spam than Jay Allen’s excellent MT-Blacklist? As the name suggests, MT-Whitelist would only allow approved URLs to be posted in the comments section of your blog. You could begin with a starter batch of okey-doke URLs and then, with each comment using a new URL, you could choose to add new ones to your list (or not). Given the relatively limited number of different commenters the vast majority of blogs attract, this might be a pretty simple answer to the comment spam problem.
(Yes, it’s Friday night, and I’m at home writing about comment spam solutions. But I can be wild and crazy, too: I just got home from a shopping spree at the book store. My life is a nonstop party.)