This is just the beginning, but the guys at GoogleEarthHacks have created a game using Google Earth. This is a war game which they claim is largely text-based. The object is to grow armies which you place at various cities and try to hold the cities. There is a GE KML file 
which lets anyone view the “action” (more like a status) of the game.
This is the first of many games I’m sure will be created using GE as a visualization tool. I expect to see more direct, or interactive, use of GE to play a game soon.
It wouldn’t be too hard to draw the polygons for Risk… :^) Hmm, something for my weekend.
beautiful. im hoping for a simple air combat game. the g-force navigation (ctrl-g) is way too much fun.
Don’t forget about http://EarthContest.com too