This appears to be broken with newer versions of WordPress and/or Buddypress. Enabling the plugin makes my entire site throw 500 errors. I had to disable the plugin via the database.
I’m looking to try and do the same thing. SI CAPTCHA Anti-Spam does work with Theme My Login.. I’d really prefer to use WP-reCAPTCHA, but they don’t appear to play well together. It doesn’t display on the Theme My Login registration page, but registration will fail for not submitting the captcha.