I would say Fun is only for small applications and really cool ideas to impress other people. To make them laught or to wonder how it works. There was a 3D simulation application on youtube last year from a student. He never released the code, but he used the front camera for headtracking and adjusted the screen depending of the angle to create a 3D environment inside the N900 screen. It was not a game. It was more like a tech demo. And perhaps more cooler than a iBeer Application... I saw also something like this using a Wii-Mote from Nintendo. Edit: Perhaps someone could create a Fun application that combine Front and Back camera - so you see a simulated 3D environment (using headtracking and the front camera) and inside this 3D environment you can see your hand that is recorded with the main camera.