WordPress has much better spam filters than Movable Type, but you can still get hit hard. Considering that we filter email spam effectively I was thinking about applying the same kind of logic as SURBL but to blogs. Someone beat me to it !! 
John Sinteur’s plugin seems to do exactly what I had in mind…
Blog Spam
How is this method working out? I was getting targeted extremely heavily, I logged on one morning to find every single post on the site had been spammed, had to run an sql query to clear them out. I’ve added in this: http://coffelius.arabandalucia.com/index.php?p=9 – and haven’t had any spam since. However, it does annoy users having to type in a code so I’m interested to see how you got on. Cheers.
Dave
The SURBL filter works quite well, but you still have to delete all the “pending” comments which is a PITA. I’ll have to look at the link you posted and try that, because I’ve got better things to do with my time than removing junk from the “pending” queue 🙂
I see you added in a code like mine. It seems the easiest solution because as as you said; manually trolling through the pending queue is not good for the brain. Good luck. 🙂
Dave
No choice really 🙂
Life is too short etc
Michele