It seems that the white iPhone 4, launched less than a week ago, has more differences from the black one. Apart from being thicker by 0.2 mm, it also has a more recessed camera lens and different-looking proximity sensor.
It was namely the back camera lens that is said to have caused the nearly one-year delay of the Great White. Reportedly, there was an issue caused by the white semi-translucent glass and the flash. This must be the reason behind the more recessed 5 MP camera lens.
The proximity sensor has been changed too. Maybe it's to address the issues with the semi-translucent white front panel as well. We are yet to learn if there are any changes in the camera quality or in the proximity sensor performance.