There may be an actual hardware reason why Siri will never come to older Apple hardware. The iPhone 4S, iFixit discovered, has an IR sensor to let Siri know that you have held it up to your face. And it almost never switches off.
Previous iPhones also have an IR sensor up by the ambient light sensor, but it only kicks in when you're on a call or using a VoIP app like Skype. It then detects the proximity of your face and cuts out the screen if you get too close, preventing accidental screen-touches with your handsome, chiseled cheekbones.
The 4S has a new IR sensor design, and it beams out IR light the whole time the screen is powered on. This is, iFixit presumes, to let Siri know when you hold the phone to your ear and issue another instruction.
The question is, is this new sensor essential to Siri's operation, or could the old iPhone 4 IR sensor be tweaked to light up more often? Perhaps the new sensor is an ultra-low power version designed to be on almost all the time? If anyone is running the hacked version of Siri on their older iPhones, let us know if Siri detects you when you hold the phone to your ear.
Little Sister Siri [iFixit]
