Parallel Kingdom is a new GPS based online role playing game for the iPhone or for phones running Google Android. The game creates a virtual world on top of the real world using the GPS inside your phone.
In the game your real location is visible to the people playing the game near you. If you can see them, they can see you and either of you can attack or chat with the other.
Presently the game is only available in the USA - but anyone can watch the video: