Same problem. Got my 920 two days ago. Had problems and needed to soft reset a couple of times on first day. Hoped it was just teething. Today, after loading skype, it has locked down completely....tile screen is on but unresponsive. Pushing power button switches off screen. Pushing it again turns on lock screen which is also unresponsive. Soft reset just returns to unresponsive tile screen. Using camera button just activates camera. Tried to hard reset using power cable but it does nothing.
Will be returning the phone tomorrow. Pretty fed up.