Overlaying real world and game

I had thought of this sort of concept a while back, but it's interesting to see it being applied:

Using an iPhone or Google Android cell phone, Parallel Kingdom knows of a users physical location enabling a virtual character to move around in that area of the virtual world. Users can attack, trade, or mingle with people around them, and can pick on friends, build kingdoms, go on raids, wage war, or establish economic empires.

Parallel Kingdom: Role Playing Via iPhone or Android With Location Mashup

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