For those of you who may have a blog running movable type, there is a new tool available to help you in the fight against comment spam. It's called MT-Keystrokes.
Basically, it checks for user input by way of keystroke before it will allow a comment to be posted. This should work because currently most comment spammers do not enter their messages manually. They have programs that will detect the comments.cgi file and post data to that cgi file, automatically posting without hitting any keys on the keyboard. Users of your site, on the other hand, actually sit there and type out their comments, so they should be unaffected.
I'm sure there will be a way around this for comment spammers, but for now it seems like a good idea. I'll test MT-Keystrokes out and post some results in a few days.