In a blog post from iFixit it’s noted that the iPhone 4S features a tweaked proximity sensor, basically an infrared LED capable of detecting how close the user’s face is to the screen. iPhones have always incorporated proximity sensors that prevent accidental input by turning the device display off during phone calls (when the phone is raised to the users ear), but Siri’s “Raise to Speak” feature requires that the device be ready to sense proximity to the user’s face at all times.
The iPhone 4S has a neurotic tendency of always wondering how close your face is. As long as the screen is activated, that IR sensor will be shining brightly. Siri is ready and waiting to answer her master’s beck and call at any time. So whenever the screen is active, the proximity sensor is active too. Thus, whenever you raise the iPhone 4S to your face, Siri is ready to take orders.
In a YouTube post, iFixit shows how the iPhone 4’s proximity sensor is only activated while on a phone call, while the sensor on the iPhone 4S is active whenever the device’s display is on.
Ongoing efforts to port Siri to other iOS devices has given hope to enthusiasts eager to port Siri to previous iPhone models, but while the Siri port may eventually prove feasible, the full experience will only be capable on the iPhone 4S with its tweaked proximity sensor.