Quite a while ago I wrote a spamassassin plugin for movable type and have been using it with great success for quite a while.
Just recently I had to move my blog from one machine to another and they had incompatible Bayes database types. This meant that the Bayes database needed to be started from scratch again and I was quite worried that this would take a while. Luckily for me I was off on holiday for 2 weeks which meant I had no access to a PC to watch my comments fill up with the usual spam. On return from my holidays I then used all the spam I had received to teach the Bayes filter again.
I have been quite surprised at how little spam I have received considering I have added less than 200 spam entries to the database. The only time I get new spam now is when I ping sites when I have added a new entry which is a good indication these sites are being spidered for recent entries etc.