Many have been asking about if Siri, which is a flagship feature of the iPhone 4S, will also work for other iOS5 devices. The answer is not clear until iOS5 is pushed out October 12th, but reports and Apple Support have confirmed:
Yes, Siri will work for all iOS 5 compatible devices. However, Siri is in beta for the time being, and the beta is first available for the iPhone 4S.
Apple Support assures that Siri will work for devices compatible with iOS5. The original Siri was compatible with iPhone 3GS and iPhone 4, so this iteration should still be compatible. The A5 processor may help with voice recognition speed, though.