I forget what game it was, but they showed the rear touchpad being used to climb a vine, which on the surface isn't very intuitive but if you can climb, direct your gun left or right, and shoot, all at the same time then it becomes something good.
The trackpad is the same size as the screen, so your hand are already in position to use it. Even allowing for the possibility to use it with the face buttons, left analog, and one shoulder button, all at the same time. So there are a lot of possibilities for use. So I agree with Ian that it could become a standard, and with the other person who said it would make sense on the PSP2-3D.