Apple has confirmed Siri will not be supported on older devices, in response to a developer’s bug report submitted to Apple. The developer suggested Apple should make available a special build of iOS 5.0.1 to include Siri on it as a paid option. Apple apparently responded to this with:
Engineering has provided the following feedback regarding this issue:
Siri only works on iPhone 4S and we currently have no plans to support older devices.
Siri will remain exclusive to the iPhone 4S–that’s a no-brainer. Siri is the one ‘major’ feature that separates the iPhone 4S from other iPhones. It’s similar to how only the 13″ MacBook Air has a SD slot, versus the 11″ Air. Users are put in a position to pay more for the 13″ if they want an SD slot. When it comes to the iPhone 4S, you must pay to play with Siri. There are no other options.