An unofficial port of the iPhone 4S' Siri voice recognition system to the iPhone 4 has been revealed, though it's unclear whether Apple will ever enable the speech control tech for the older handset. Currently the port - the handiwork of developer Steve Troughton-Smith and 9 to 5 Mac - is capable of loading the Siri interface and controls; however, since Apple's server-side processes are integral to Siri's interaction with the iPhone hardware, right now any spoken commands won't be followed.
There are also some significant speed issues, which are evident in the video demo below. Siri lacks the necessary iPhone 4 GPU driver since that wasn't present in the iPhone 4S's binary cache, and so the whole interface is a whole lot more sluggish than what you'd see on the iPhone 4S.
On the flip-side, however, with that GPU driver in place, it's entirely possible that the Siri system would work just as well on the iPhone 4's A4 chip as it does on the iPhone 4S' A5. Apple's motivations for limiting it to the newest version of the smartphone could well still be out of consideration for performance, but it's also highly likely that - as one of the flagship features for this fifth-generation of handset - it's being preserved as a differentiator to try to encourage people to buy the iPhone 4S rather than the cheaper, older model.