Some folks have been reporting problems with the captcha response during registration. There is definitely a small bug in how incorrect responses are handled.
First, the response is case sensitive, so be sure to enter the appropriate capitalizations.
If you do get the error message indicating that your response was incorrect:
1) Go back to the registration page. The image you see for captcha is no longer valid, so...
2) REFRESH the page. With that, you should see the latest captcha image.
I hope this helps, I'll be fixing this shortly.