[dnautics]

How about instead of a 1:1 touchpad on the back, which could have problems, Put a trackpad on the back of the phone, but have it just be a small square. So it wouldn't be a "full-fledged" touchscreen, but a small touch sensistive section. You could also have a discrete capacitance detecting sliver on the side of the phone so that it's only active if you're physically holding the phone. Clicks would happen using a button.

[ATLobotomy]

Why does it have to be on the back? Some old Android phones had a trackball (or an optical sensor like on a mouse) to be used as a sort of cursor. The Palm Pre had a small touch sensitive area below the screen that could be used for OS specific gestures (much better integrated than what Android used the trackball for IMHO).

All of these ideas are pointing to the issues in the use of a touchscreen as input (imprecise, blocks screen during use, etc), yet for some reason they keep getting used (and even taking over regular buttons).

I hope that this decision is being tested by the companies HCI departments, but I worry that marketing is deciding that changing the input would be too much of a risk (or cause fragmentation).