Apple has made it clear that Siri will not be available on any other iOS devices besides the iPhone 4S. iFixit believes this has to due with the device's unique proximity sensor. The iFixit team discovered whenever the screen of the iPhone 4S is activated the proximity sensor turns on.
The proximity sensor in other iPhone models is basically an infrared LED light that lets the device know when a user's face is near the screen. This allows the iPhone to switch off its touch interface when someone is making a call. iFixit believes the sensor inside the iPhone 4S switches on constantly so users can easily access Siri.
"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."
According to iFixit, the tweaked proximity sensor is what sets the iPhone 4S apart from other iPhone models, but some people believe a software update could enable the iPhone 3GS and iPhone 4 to work the same way. It just isn't a very strong excuse to keep non-iPhone 4S users Siri free.Developers have proven the voice assistant can be ported to the iPhone 4 and iPad, but neither device is set up to communicate with the Apple servers which is required for Siri to work. If the servers can be tricked into communicating with other iDevices, and the proximity sensor can be tweaked, there still may be hope that iPhone 4 users will be able to enjoy the total Siri experience in the future.
UPDATE: iFixit told 9to5Mac:
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The IR LED definitely appears to be different on the 4S when compared to the iPhone 4, but we don’t think that the hardware change is causing the light to be continuously lit. That functionality is built into the software system, since it’s the software that tells the light when to turn on, and when to turn off.