Siri has been in widespread use for four months, but so far Apple's "personal assistant" is still only available on one device, the iPhone 4S. We speculated that there weren't any technical reasons Siri couldn't work on some of Apple's other devices, and the jailbreak community later proved us right by porting Siri to the iPhone 4.
AppleInsider did some digging and discovered there may be a technological reason Apple's kept Siri an iPhone 4S-only feature: noise reduction. The iPhone 4 incorporates noise reduction circuitry from a third-party vendor called Audience, and that circuitry lies separate from the A4 chip on the iPhone's logic board. The newer processor in the iPhone 4S (and possibly the iPad 2) incorporates a newer version of this noise-reduction circuitry within the A5 chip itself, reducing overall cost.
Audience's noise reduction chip works similarly to how the human brain processes audio. By sampling audio from multiple sources (the iPhone's main microphone and the noise-cancelling mic), the Audience chip is able to filter out background noise and deliver only the user's voice, just like how your brain filters out noise in a crowded room to focus on a person talking to you.
The newer noise reduction circuitry in the A5 chip is better at "far-field" noise reduction than the circuits in the iPhone 4. Essentially, the iPhone 4S can achieve the same or better noise reduction when held at arm's length that the iPhone 4 gets when held directly in front of a user's mouth.
The implications for Siri use are obvious -- because of its less advanced noise reduction circuitry, Siri wouldn't function nearly as well on an iPhone 4 in an even moderately noisy environment unless you held it up to your ear and talked directly into the microphone. Despite having an A5 processor (and possibly including the newer noise reduction circuitry), Siri might not function well on an iPad 2 either, since the iPad 2 doesn't have a noise-cancelling microphone.
Apple's product perfectionism often leads to scenarios where features that might technically work on a product wind up excluded because they don't work perfectly. I've run into this a few times with older gear; my old PowerBook G3 had no technical barriers to running OS X Panther or Tiger, for instance, but because it didn't run anything newer than OS X Jaguar well, Apple artificially restricted the device to Jaguar. Similarly, jailbreakers discovered ways to get video capture working on the iPhone 3G, but the results were rather lackluster compared to the officially-supported video recording on the iPhone 3GS and above.
Even if Siri technically works on Apple's older iOS devices, if its performance doesn't work to Apple's satisfaction, we may never see Siri ported to the iPhone 4 or current iPads after all.