Reports indicate Siri computes primarily in the cloud (the whole sound file is sent), and that it's not on iPhone 4 for other reasons (e.g. reduce server load, increase 4S sales)
That is true, as people have ported Siri to the iPod Touch and iPhone 4, and it is performing exactly the same. Therefore, they either emulated a whole chip (highly unlikely) or everything is processed in the cloud/on the A4/5/#.
