I suspect that Apple, Samsung, etc. have thought of this, so why aren't they doing it? (After all, the PS Vita has had a back touchpad for a while.) The first thing that comes to mind is that people hold their phones with the back. Accidental clicks would be very difficult to avoid. In contrast, the PS Vita is larger, held with both hands and has non-touchpad areas of the back where you can grip it, as I understand.

I'd still like to see someone try it.