According to the Back to My Mac tab in the MobileMe, "Back to My Mac isn't working properly because it requires a router that supports NAT Port Mapping Protocol (NAT-PMP) or Universal Plug and Play ...
and spotted a feature known as NAT Mapping (inbound/outbound)on their spec sheet. Then I go to WikiPedia entry for NAT Port Mapping Protocol (NAT-PMP)and read that "It has no built-in authentication ...