Not "predicted" of course is how it would work, and how it would filter out things like holding the iPhone for normal use vs. deliberately initiating a multitouch gesture on the back of the case.
Again, with Apple anything is possible, but is this desirable? Can you see Apple doing it?
The Palm Pre and Pixi have a multitouch area below the screen to enable consistent gestures for "back" and to bring up the app launcher. Is that preferable?