The intent of the MobileMe/BTMM here was to enable you to have an Internet hostname associated with your router. You'd use that instead of the 173.x.x.x public IP address. The beauty of this is that a) you have an easily-remembered name to use instead of a bunch of numbers, and b) the router keeps Apples servers aware of its current public IP address at all times.... even if that address changes.
If you have a computer that is on 7/24 or at least frequently, you can set up free DynDNS account; there is a software update client that can be installed on the computer, and this will keep DynDNS's servers up to date if you public IP address changes. The only reason I mentioned MobileMe is because AirPorts don't support DynDNS - the majority of other SOHO routers do. Don't install the updater on a mobile computer - do it only on a computer that always stays at home.
If you have a computer that is on 7/24 or at least frequently, you can set up free DynDNS account; there is a software update client that can be installed on the computer, and this will keep DynDNS's servers up to date if you public IP address changes. The only reason I mentioned MobileMe is because AirPorts don't support DynDNS - the majority of other SOHO routers do. Don't install the updater on a mobile computer - do it only on a computer that always stays at home.