On the iphone 4s, what is that black thing on top of the earpiece/speaker?

I'm trying to debate what color iphone 4s to get since i'm getting one tomorrow. I noticed that on the white iphone 4s there's a black line above the speaker/earpiece, and that isn't there on the black one. What is that?

Added (1). I'm referring to this

They all have it, it's a motion sensor. You just can't see it very well on the black.

Its a proximity sensor, that turns off the screen when you put it up to your ear.
all iphones have it.

That "black line" is the ambient light sensor it detects the amount of light in the room/surrounding area that tells the phone to become brighter or dimmer if you have auto brightness selected in the brightness & wallpaper options.

The black one DOES have one it is just blended into the color of the phone because the phone is black and the ambient light sensor is black

Motion Sensor.

When you are on the phone/talking, it uses the camera to turn off the screen. Trust me, try it yourself.

The black thing could be some sort of sensor that detects movement or lighting conditions.