MobileMe has never been a domain name registrar whatsoever. You can (soon no more) point your personal domain name to MobileMe, but not register/host with MobileMe.
In other words, your personal domain name HAS to be registered with such registrars as NameCheap, GoDaddy first in order for it to work with MobileMe. You can't simply point an unregistered domain name to MobileMe.
Hope things are clearer now.