Hi chad. There are two IR sensors on the bottom (one on each edge) and an IR sensor on the top right edge.
If you go in a pitch black room and turn the display off (tap the power button not turn the phone off completely) you will notice them. Pretty neat stuff.
I have a feeling that the three "sensors" are actually iR emitters, just like a TV remote has. I think the actual sensor, or receiver, is near the speaker, but doesn't light up. That would explain why gestures are only noticed toward the top of the phone.