When I signed up today some moments ago, the CAPTCHA failed to come up and attempting to submit the form resulted in nagging about the missing CAPTCHA.
So I poked it with Firebug and noticed that the
https://api-secure.recaptcha.net/ connection was failing. When I navigate there, I get an SSL error that says "The certificate expired on 05/04/2009 08:32 PM. The current time is 05/03/2011 07:39 PM".
A certificate that expired in 2009?!
Some digging brought up
this Stackoverflow thread, which says that the new URL is
https://www.google.com/recaptcha/api
I changed that with Firebug and re-launched the CAPTCHA init function and it worked. You'll probably want to fix the registration page!