A Difference between the new white iPhone 4 and black one has been spotted by one of MacRumors readers, he noted that white iPhone 4 is ever so slightly thicker than black iPhone 4.
white iPhone user :
Just picked up the white iPhone 4 and realized it doesn’t fit into my Incase slider case. It appears that Apple has increased the size of the plastic that borders the glass on both sides by about 1mm.
The folks at Tipb have calculated the difference at 0.2mm. They also tested some cases which seemed to work for both white and black iPhone 4.