In a lot of these responses people seem to be getting stuck on the idea of using an iPhone as a controller for some reason (you'd be looking at the screen ect) and I dont quite understand why people are having difficulty seeing the idea of using the Apple TV and iPhone as a games console?
Don't think of interacting with the iPhone as you do now, instead the iPhone could "show" a standard joypad so you would be controlling the game with the iphone but looking at the TV, or perhaps the screen isnt used at all and the iPhones accelerometers are used as you tilt the iPhone (driving games perhaps). More inventive developers could even go as far as using both the screen on the TV and the iPhone. Using the example of Driving again the TV would show the driving, iPhone would work as the controller and the iPhone screen could display switches/buttons/fuel levels/boost levels ect or even the rear view mirror. Or perhaps looking away from the screen could be made to be a big part of the game, for example a horror game where you would walk around but in order to open doors ect you would have to look down at the iPhone to do a puzzle, but youd have to keep looking at the TV to make sure no-one is sneaking up on you.
All these things would be possible, you just have to think outside the box and if apple really get behind it could really become game changing like the wii was (and the natal ect arent)