A nice feature of the iPhone is of course the built-in accelerometer. With this the device's movements can be measured in every 3D dimension and in regard to itself (rotation). A lot of fun applications are made for this at the moment.
I think the ones which are the most fun still have to come, namely when you can transmit the movements through Safari, the iPhone's internet browser. Then a third party can view along from a distance and react automatically. If you also turn on your camera, the image is complete. Then brands can reconstruct 3D images and determine exactly in what kind of environment you are and assist you. But just measuring the location change of the iPhone would be a step forward. Then you can determine the location even better than with just GPS and you can see, for example, how many centimetres you're away from someone else('s iPhone). It shouldn't get any crazies... But it will.