I’m at last attracting spambots! Hopefully, that means some people are reading, too.
I’ve added CAPTCHA to the site using Bot Check. Although it does a less thorough job of scrambling the image than the AuthImage plugin, I was uncomfortable with the number of changes I had to make to WordPress to make AuthImage work. (If you install Bot Check, be sure to read down in the comments for a change you need to make to it for Wordpress 2.0 and later.)