Using (Electronic) Games To Interact With The Real World

Lately I've been really interested in stuff like Geocaching/GPS technology and how they could be used within the the context of a game set IRL. With most phones in this day and age having some sort of location awareness, it seems like a no-brainer. Services like Facebook Places and Foursquare are becoming really popular.

Also, it would be interesting to subvert the traditional idea of handheld gaming. More often than not, whenever I see someone using a iphone and DS etc. the purpose is usually to block out the world. Of course it's much different in japan where ad-hoc gaming is the new hotness, but it would be cool to play a game that allows one to become more aware of his surroundings and the people within them.

I've had a couple of ideas for games, my favorite being a city-wide game of hide and go seek using google maps to mark out the play area and the game's "app" showing google street map images nearby other player's locations.

What do you think? Have any good ideas? Are location aware games viable design/cost wise?