Domain Mapping Not Working

I am trying to map a domain but it is not taking. I have 12 other blogs setup on this WPMU instance and they all have URLs. But when I try to setup this new one from the Domain Mapping form, I hit the add button and nothing happens. I mean it refreshes the page but there's no evidence it took. I then try to go to the site for that URL and it just sends me to:

http://{mywpmuinstance}/wp-signup.php?new={mydomainname}

with a message that says "registration has been disabled".

Any ideas?

Thanks.