On the Blackberries, when you put them into the free holster provided by blackberry, it automatically locks your phone and shuts the display off. I'm guessing this could be one application for a touch sensitive case.
Another, could be that the back side of the case is touch sensitive to possibly accommodate one "rumored" feature of the 4th generation iphone, which some people say will have touch sensitive gestures on the back side of it similar to the new mighty mouse.