From MacRumors.com: Commuters: Get Extra Sleep with iNap

Ever wanted to get some sleep during a train ride, or a quick powernap on the bus to work? You either hoped to wake just in time not to miss your station, or set an alarm to wake you far too early... Let iNap wake you when you get close to your stop!

The application uses the iPhone GPS to wake you when you arrive at or within a certain distance before arriving at your destination.  Pretty cool.