It turns out that Apple's Siri voice assistant is enabled by special noise reduction hardware called earSmart, wired right into the Apple A5 processor. The technology is made by Audience, which has an earSmart-less chip in the iPhone 4. The difference is, the iPhone 4 chip was designed to reduce noise when speaking directly into the microphone (like when you're on a phone call), while the chip in the iPhone 4S is designed for a wider field of sound, so you could talk to your phone at arm's-length (as one tends to with Siri) and still benefit from noise cancellation. The bundling of earSmart with the processor explains why the A5 chip was so much bigger than expected, even after taking into account the second core. It also means that, if you're still desperately clinging to the hope that Apple will someday update the iPhone 4 with Siri functionality, now's the time to let go.
Siri was obviously a defining feature of the iPhone 4S, which was sorely needed considering all of the cosmetic similarities to its predecessor. Apple was even willing to throw in an extra infrared sensor to optimize the Siri experience. Now the assistant is poised to leap onto Apple TV and the iPad, assuming the earSmart is included on those devices too. Now, if things could get a little more optimized on the software side so that Siri doesn't double your data usage, then Siri would have all of its bases covered. It's worth noting that earSmart also licenses out to a bunch of other manufacturers, so you can expect voice control on other platforms over time. Just, uh, don't expect the Android alternatives to be blowing Siri out of the water just yet.
While this doesn't leave a lot of hope for an official port of Siri to the iPhone 4, you can always try some warrantee-exploding hackery... Head on over here to see some of the devices that currently have earSmart included.