From Forbes:
Participants meet on the street (using Google Maps) and "shoot" each other with their phone cameras. Those with the highest number of "unique kills" earn points and bragging rights. As the name suggests, players can form "armies" of up to 100 people.
The game is the brainchild of 23-year-old Chicago entrepreneur Peter Wojtowicz, who goes by his screen name, Peter Whatanitch. He describes WiFi Army as "augmented reality," meaning it brings aspects of virtual reality, such as the concept of an online army, to the real world. Its blend of GPS, wi-fi networking and Google applications points up Android's potential to reshape mobile phones and the way people use them.
Sound familiar?