From a pure marketing standpoint, it makes perfect sense that Siri works only on 4S. Were it not for Siri, there would be very little incentive - almost none - to upgrade from 4 to 4s. But that's not the same thing as saying it can't physically be done, or it needs a particular chip to do it.
In fact Siri is simply a clever extension of technology that's been around for years, so it's more of a breakthrough in imagination than a breakthrough in technology (and no less worthy for being that).
I'll bet my proverbials that there will be some work around or fix which, though not to Apple's taste, enables users of 4 or 3GS handsets to engage with Siri.
