iLounge is reporting that Apple has included a second proximity sensor on the iPhone 3G to improve the accuracy of face detection.
On the first-generation iPhone Apple included a proximity sensor just above and to the right of the earpiece, when activated the sensor turns the iPhone's screen off preventing accidental screen presses and unnecessary battery waste.
As a result of the second sensor to the left of the earpiece many case manufacturers have increased the size of the opening around the earpiece.