The "slots" you see on these phones are not actually slots actually. The bigger one, which can be seen on both phones is the actual speaker for calls and such.
The smaller, is the proximity sensor. You can't see it on the black because obviously it blends better. However, if you were to take a bright light and shine it on the top of a black iPhone 4, you'd be able to see it.
I have not seen these phones "in person," but pictures of the white one show two "slots" at the top, while the black one appears to have only one. Am I just not seeing the black one properly, or are they different? If they are different, why? Thanks.