AppleInsider is reporting that Apple engineering has officially stated that there are no plans to bring Siri to old iOS devices.
I personally never expected Siri to come out on older iPhones. Sure, I would have loved for my wife's "old" iPhone 4 to get Siri. That does not make any sense for Apple though. It's an amazing selling point for the iPhone 4S. I still think that the iPhone 4S was specifically designed to work best with Siri and that the performance on older hardware is likely to be worse (i.e. more battery drain to do the same function).