Earlier quoted context omitted.
This is because the iPhone 4 is missing a newer version of the crucial noise reduction chip: http://www.tuaw.com/2012/02/06/siri-may-be-iphone-4s-only-be... The fact that Apple supported iOS 6 on the 3GS is proof that they're pretty serious about backwards compatibility.
That is not true. Siri was available in the App Store from a third party for at least a year prior to its reintroduction in the 4S, and ran on a 3GS just fine. There isn't a technical reason for it not being available on earlier models. http://techcrunch.com/2011/10/04/the-original-siri-app-gets-...
And especially given Apple's need to work across multiple languages we don't know if a more sensitive microphone was required.