I doubt the rumors of the back being touch sensitive are true, although it is highly possible.
If they make the back casing touch sensitive for anything important then they will be potentially killing the iPhone case market as a case would then cover some of the controls. Having said that, the reason I still think it's possible, although highly doubtful, is they could always make it touch sensitive, but the controls are not for necessarily important tasks, ie finger swipe to change tracks etc, and hence only people that want to use them need to.