BuddyPress’s registration page and process don’t use the hooks WP-reCAPTCHA is expecting.
Here’s a small (~40 lines) plugin to make the reCAPTCHA field show up and be processed during registration. This has only been tested on single-site installs, and probably won’t work (or will mess up) a multi-site install, because that will trigger different behavior from WP-reCAPTCHA.
