I would say a back screen
The kind you can use on Nokia(?) phones to know if you are taking a picture of yourself or of the ceiling with the phone camera.
...Except that now that the iPhone has a front camera, it seems kind of useless. For taking high-quality pictures of you, maybe? (the back camera is better than the front one, right?)
Of course, it is also more useful on flip-open phones, because it acts as an outside screen when the phone is closed. Maybe Apple will have flip-open iPhones, now?
A touch pad might just be possible, but not a track pad - way too small for the appletastic pinch-and-sweep moves.
There is also a good chance that it is a prototype that will never see the day.