The CAPTCHA experiment was a failure. I didn't get any blog spam, but I heard from a few people who tried to post comments and failed (other's succeeded). In the end, I didn't feel like debugging it, or worse driving people away, so I determined to abandon it.
Still, I need a way to combat comment spam, so I went to a simpler, text-based CAPTCHA. This isn't as flexible as I'd like, but it's likely to do the trick. The main problem with it is having to edit code. When I update MT, I'll have to reinstall it. Of course, that's not the first time I've done that.
If you use this solution with MT 3 or above, you'll find that Six Apart changed how comment fields are generated in the system templates, making it impossible to add the spam text field. To fix this, use this replacement for <MTCommentFields>. This has the added benefit of putting the new spam text input field in just one place so that you can edit it more easily if needed.