One that works without 3G
It would be nice if the ap worked off the gps even if it lost the cell service. Some aps stop working completely if they lose cell service. You could design an app that allows downloading additional maps and data and updating while cell service is available, and just show the functions that are unavailable due to cell loss as unavailable while allowing the gps only functions to continue to function. Store the maps on the sd card instead of using the network and you will still have it available. I think Mapdroyd does this for the android system. Speaking of which, don't forget the Android market, it is growing really fast. I got a Motorola droid and am completely hooked.