According to an anonymous Gizmodo reader here [ http://gizmodo.com/5570963/apple-white-iphones-more-challenging-to-manufacture-than-black http://gizmodo.com/557

According to an anonymous Gizmodo reader here [ http://gizmodo.com/5570963/apple-white-iphones-more-challenging-to-manufacture-than-black http://gizmodo.com/5570963/apple-white-iphones-more-challeng... (see update 2)] the display glass is not painted but screen printed, presumably so it can be translucent for the sensors beneath it. It seems that Apple want to achieve both the required translucency and an even, saturated white, which is quite tricky.