The apparent reason for Siri being 4S only is the microphone. In the iPhone 4, they used a secondary microphone to sort out the background noise. This two part microphone is key to being able to make Siri work properly and is not on any other phones. The iPhone 4 does not have the A5 which is also required. And after two years of work, they still release Siri as a beta program. Apparently all the tweaks needed to make Siri work with the iPhone 4S were also a big deal, so you will not see Siri on any backward products. OTOH, the iPad 3 is strongly in the running for such a setup, in my opinion.