Reported by a user using WP 3.1.3
On the register user page, it appears to remove SI CAPTCHA Anti-Spam form fields using jquery manipulation of the form fields
PS view the HTML source of the register page, there is an error:
Warning: call_user_func_array(): First argument is expected to be a valid callback, 'fbregister::register_form' was given in wp-includes/plugin.php on line 395
Looks like your facebook register plugin is causing that error.