In the course of researching Movable Type anti-spam techniques, I came across a Bayesian filter plugin by James Seng. It looks promising, but after reading this entry at Al-Muhajabah's MT Tips blog, I've decided to hold off. Apparently you need something like 1000 comments before it 'learns' enough to be useful. I have about 100 comments, so maybe I'll try it in a couple of years