VentureBeat says that Woz get their hands on a piece of white iPhone from Foxconn. After iPhone Modding 4, Woz realized that the images, as if taken by the cellophane saw.
Wozniak said that its iPhone white 4 piece causes problems with proximity sensor (although I think it refers to the ambient light sensor, so that you may have similar problems in the camera to see light leakage photo.) Then confirmed that Apple have problems with the solved white iPhone 4, and so soon.
Macotakara report also proposed that a new material developed by Japanese painting finally solved the production problems with the iPhone white 4.