If you have been wondering how Siri reacts so quickly when you "raise to speak", the answer is a tiny infrared LED that's added into the proximity sensor in the iPhone 4S. iFixIt discovered the sensor after a second look, determining that it shines brightly whenever the screen is activated. The light isn't visible to the human eye but it's there. And it's constantly letting Siri know how close your face is to your iPhone 4S.
It does raise a few questions, though. IR is fairly light on battery drain but every little bit extra does add up. Also, iPhone 4 and other older models lack the extra IR sensor, making it seem even more unlikely Apple will ever offer an official port.
If you're interested in seeing how it works you can check out iFixIt's video via the link below.