One previously undiscovered change in the iPhone 4S versus the iPhone 4 is a new infrared proximity sensor that enables the raise-to-speak feature used by Siri voice recognition.



From Apple Insider:

The teardown experts at iFixit revealed on Wednesday that the proximity sensor in the iPhone 4S has a newly modified infrared component that's enabled whenever the handset's LCD screen is active. Previous iPhones only enabled the proximity sensor when users were on a phone call or utilizing an application that required users to hold the device up to their ear.

Now, with the iPhone 4S, the infrared sensor is always working while the handset is in use, allowing users to utilize the raise-to-speak feature available with Siri voice recognition. The sensor allows the touchscreen on the iPhone 4S to automatically turn off so that actions cannot be taken by pressing the screen when either Siri is being used or a phone call is being made.

  Siri raise-to-speak feature uses modified iPhone 4S proximity sensor