The iPhone 4 was supposed to launch in two colors: white and black, but at launch only the black iPhones made it to store shelves and the white iPhone keeps getting delayed. According to several sources, it's because the white iPhone 4 leaks light.
The front and back of the iPhone 4 are made of a kind of ceramic glass, and the white version of that ceramic lets light through, especially at the edges, according to a source for The Street. The black iPhone casing is opaque enough that it wasn't a problem, but the white iPhone 4 continues to leak light from the screen all around the device.
The light leakage doesn't affect the iPhone 4 performance; it's mainly an issue Apple is addressing because the company puts so much emphasis on aesthetics that they can't let a cosmetic issue like this go.
BGR confirms that light leaks are the reason behind the delay, adding that Apple is having trouble stopping the leaks. Apple has used a silkscreened paint on the back of the device, but can't get the paint right on the front because it needs to be opaque enough to stop light but thin enough to allow the internal proximity sensor to work.
Apple is also struggling to keep the white paint from discoloring due to heat or from ingredients in the paint.
Another BGR source says these issues are mostly resolved, though, and white iPhone 4s should be shipping in the "near future." Still, one wonders why this much effort wasn't put into perfecting the antenna instead of the color of the phone.