It is only by a freak chance that I found an email reply from someone in December about a skinning problem they were having where they mentioned they that they couldn't register. I never saw the email originally because it got moved to a sub-folder and marked as read in the middle of a load of other emails.
It seems that the upgrade to Gold added some checking for the amount of time spent reading the ToS on signup. Since we do the ToS slightly differently then this new checking was failing and everyone who tried to register couldn't get past the ToS page.
This is now resolved with a quick work-around and registrations should now be functional again. Sorry to anyone who tried to register but didn't tell me about the problem.