Gravatar now requires having an account WordPress.com - current users will find that they are already registered there but new users will need to register on WordPress first.
Thought I'd give you the heads up on this slight change to Gravatar in case you're not aware of it; maybe some info on this site might need to be updated to reflect the new set-up.
It doesn't actually look like a problem however. I've read it to say that new users will sign up with a "wordpress connect" service - which I assume will be available on the Gravatar page.