I have no doubt that some nutter, somewhere, will want to test the resolve of the flight crew by killing some cabin staff and maybe even some pax. But they won't be silly enough to kill a lot of them because they know that they'll need bargaining power on the ground, after the plane makes an emergency landing.
I suspect that the reinforced cockpit doors will be effective, but I'd hate to be one of the casualties in the cabin that proves the point!
