This, friends, is the future of mobile apps continuing on its march. Augmented reality--the ability to overlay various data sets on a real-time view of your surroundings--will change the way location-based data gets presented.
Now, using the new iPhone 3GS's compass (sorry, older iPhone folks), app developers Acrosshair have put together a subway finder app for New York and London that overlays the direction and distance to the nearest station, depending on the direction you point your phone.
Yesterday it was business cards, last month was our augmented reality magazine cover (the world's first), and now, it's a handy subway finder for tourists and locals alike. The NYC and London apps are currently awaiting iTunes approval.