As far as assistants go, Siri can be rather demanding. In fact, she apparently requires that your iPhone 4S's proximity sensor be on whenever the handset's screen is activated. That's the conclusion iFixit arrived at, upon discovering that, unlike previous generations which flip on the sensor upon firing up a phone call or Skype conversation, the 4S kicks on the infrared detectors whenever the screen turns on. The change would allow Siri to detect just how close your face is as you speak to her without the handset pressed up against your face. More details in the video after the break.