It's about time that I plugged Geoloqi. Aaron Parecki and Amber Case created Geoloqi, which is an amazing mobile and web platform for securely sharing location data. They have created a few location-based games, including MapAttack which can be set up and played with mobile devices in geographic spaces:
MapAttack at WhereCamp 2011 from aaronpk on Vimeo.
They're looking into other sorts of location-based gaming applications. If you have the inclination, do please have a look-see at Geoloqi and all the amazing funa nd work things they're doing with it.