I just tried registering with a genuine email address and it told me I was Spam! Didn't get a chance to answer any questions.
Is the registration system working OK?
No idea - I'll take a look

Yes, was failing the test questions because no answer to them was given (hardly fair, since the questions weren't displayed). I've taken the questions away for now, registration will work as per normal.
The anti-bot quiz addon sounds a little extreme to me, not sure we need that just yet - would be nice to get the questions bit of registration working though (suspect it's just missing form the template).
I did a forum reinstall from fresh this morning, as last time it was an upgrade... but this didn't fix it.