This is really cool. The game NOVA by gameloft is an First Person Shooter. What's neat is that you can use the gyro in the device to look around. Check it out!
http://www.youtube.com/watch?v=F2liCSeCVuk (~2m40s)

Controlling an FPS by moving the device would be a very cool thing to try.

The video also features the AVATAR game (based on the motion picture). It actually doesn't look that bad!