Is Siri specific to iPhone 4S, or will it be a part of iOS 5? It sounded like it is only a part of iPhone 4S, but it seems like it's all software. Perhaps the iPhone 4 isn't powerful enough to do the voice recognition seamlessly?
From what I hear the A5 in the 4S has some hardware components not available before - including a hardware DSP and image processor. That's what's powering Siri and the newfangled fast-camera features.
