I can see it now. A crowded subway. Strangers jostling, trying not to stare at each other, trying not to smell the hygenically-impaired. Up pops Jimbo from his seat, arm outstretched, rectangular grey object in hand. His arm jerks to the pops of bullets.
Jimbo's fighting zombies with his cell. Dang if the angry mob of terrorist-paranoid, stressed-out commuters who jump him and pummel him unconscious in defense of all aboard didn't realize that.
Do we really need people walking around using their cell phones as guns for FPS games? Really?