SI CAPTCHA doesn't work with Bulletproof Security?

I’m using SI CAPTCHA and Bulletproof Security (BPS) plugins.

When both plugins are activated, BPS works fine, but SI CAPTCHA does not. Specifically, the CAPTCHA image and input box is displayed, but the plugin does not block improper logins…does not block logins where the input field is left blank, etc. It simply acts as if there is no CAPTCHA requirement at all!

If I deactivate BPS, then SI CAPTCHA appears to work properly…but this is strange, since as far as I understand it, BPS doesn’t really *do* anything when it is activated other than check to see if its various .htaccess files are in place. When BPS is deactivated, those same .htaccess files remain where they are.

Any ideas on what could be causing BPS to interfere with SI CAPTCHA?

It would be good to know, since BPS is one of the most common security plugins out there, and any new installation of SI CAPTCHA stands a good chance of being located on a BPS site.

Thanks for any suggestions!

http://wordpress.org/extend/plugins/si-captcha-for-wordpress/