This is an amazingly creative use of all the iPhone technologies, and New York is one of the densest places in the world so this would definitely be very helpful. It's not helpful for locals (who have the lines memorized), but for travelers who get confused and turned around in the maze of skyscrapers... this will be a great help.

I wonder if the technology is easily extensible to take new maps and locations. Post that openly and watch the world start creating further content... not just more subway maps, but more geographical locations (restaurants, hotels etc).

Exciting!