I doubt that the lack of Siri on the iPhone 4 is really a hardware issue. It is probably a product differentiator for the 4S. That is only a guess.
The keynote pitched Siri as a iPhone 4S feature only.
I think it'll be like when the 3GS came out. 3G didn't get multitasking. But when jailbreaking the 3G you could add that and folders. Multitasking on the 3G ran pretty sluggish though so if they get Siri to work on the 4 I'm afraid it'll be sluggish and not work exactly how it's meant to be.