I mentioned earlier that I was using Peter’s Anti-spam Math plugin to allow for freer commenting (after inspiration from Shamus). I was also using the WP-Deadbolt plugin to add a blacklist to the blog registration page. However, I left a comment at Peter’s asking if he could add the same captcha functionality to the registration page as well. He responded almost immediately and sent me a link to the new beta of his plugin, which adds the captcha to the registration page as desired and which also implements a domain-based blacklist for registration email addresses to boot! I just installed it and am really impressed. If you have a WP blog and allow users to register, give this a whirl and let Peter know if you come across any bugs.