Seems that Yahoo is involved in some self-censorship. They are banning usernames such as Allah, Osama, Binladen, and Raghead along with some sensible ones like yahoo, admin and security. Allowed are such names as god, jesus, jehovah, buddah, koran, nazi, priest, jihad, terrorist, and some fun ones like jewskilledjesus and pedophilepriest.
While Yahoo remains mum on the subject users with names such as Callahan are unable to get emails based upon their own name.