One of the design philosophies of Apple of late is that there are no on/off switches on their devices. You pick it up, you press the home button, swipe, and it’s on. In the case of the iPhone 4S and the new Siri voice recognition program, you can simply raise the phone to your ear and then talk to Siri, and it responds to your requests as if by magic.
The good folks at iFixit have a new post detailing how the iPhone 4S’ infrared proximity sensor tuns on (and stays on) whenever the screen is on, presumably to be ready to activate Siri if you want to give a command.
The proximity sensor has been present in previous iPhones in order to shut down the touchscreen when the phone is held up to your ear, thus preventing accidental input from the side of your face while on a phone call. However, it would only come on when a call was initiated. With the iPhone 4S, the proximity sensor will continue to stay on as long as the screen is active.
Of course, if it bothers you, you can always turn off this feature of the iPhone 4S. Simply go to the Siri settings in the General Settings page, and slide “Raise to Speak” to the OFF position. You lose the cool factor of just talking to Siri without pushing a button, but you may find an improvement in battery life (although iOS 5.0.1 may just take care of that for us, we’ll have to see).
In the meantime, check out iFixit’s video post if you want to see the IR proximity sensor in action.