As one of the first augmented reality apps for the iPhone, you really need to watch the videos to get a feel for how it works, but essentially, using your camera and built in compass, the app will indicate how close various stations are from your current location using your current visuals as a guide.
There are two versions, one for New York’s Subway and one for London’s underground – both awaiting approval in the app store. Have a watch to get a better idea of what to expect:
Acrossair New York Nearest Subway:
Acrossair London Nearest Tube: