Acrossair are launching what might be the first Augmented Reality iPhone App. Called Nearest Tube, the iPhone app for 3GS uses the inbuilt GPS + Compass to bring an augmented reality experience to your iPhone to help you find the closest subway station entrance! It works when held flat by showing you lines and directions to the nearest station or hold up at chest height and watch it put live markers in-front of you to follow! Very cool.

Source:
http://www.digitalbuzzblog.com/new-york-subway-augmented-reality-iphone-app/