The iPhone 4S uses Apple's A5 processor. It's bit of a brute: not only is it powerful enough to feature in the iPad 2, it also has some weighty audio processing hardware baked into it. In fact, it is home to Audience's "EarSmart" technology. Essentially, that uses a digital signal processor to remove background noise and secondary voices so phone calls sound better, and it's all built in to the A5 processor.
If you've never heard of Audience, don't sweat it: they're a start-up, but they power Apple's noise-reduction circuity. Audience did, however, recently file its IPO (it didn't get as much press as another filing, oddly). As part of that, the company revealed that its iPhone 4-era technology only works well when the phone is held near the speaker's mouth. The noise-reduction technology built into the iPhone 4S, however, is far better. It can cope with what Audience call far-field speech.
That is, in a nutshell, why Siri only officially features on the 4S. It's an app designed to be used from a distance: while the phone sits on your desk, as you're holding the handset to view a search result, or (perhaps less safely) while you're driving. The iPhone 4's audio processing abilities just aren't up to that task.
So, while hacks exist to install Siri on your jailbroken handset, or even legally, the plain, simple truth is that it won't work very well. Still, not to worry: it's not like Siri's that good anyway. [Securities and Exchange Commission viaCNET]