Ryan Cash, who posted comparison shots of the two models on his blog, noticed that the white iPhone 4 is slightly thicker than the black iPhone 4 went he placed them side-by-side on his desk. It’s a slight difference, but enough that the white iPhone 4 wouldn’t fit in a slider case form a old black iPhone 4.
It appears that Apple has increased the size of the plastic that borders the glass on both sides by about 1mm.
Hmmmmm, really interesting