Siri is one of the major selling points of the iPhone 4S. The voice-activated personal assistant isn't available on the iPhone 4, iPad 2, or any other Apple device, despite people hacking older devices to run it just fine. And that won't be changing anytime soon.
Despite the iPhone 4 and iPad 2 being perfectly capable of running Siri smoothly, Apple has decided to keep it iPhone 4S exclusive. Cult of Mac blogger Michael Steeber bugged Apple customer service until they gave him this quote:
Engineering has provided the following feedback regarding this issue:
Siri only works on iPhone 4S and we currently have no plans to support older devices.
Of course, Apple has been known to reverse course in the past. But they don't have many good reasons to do so here, as Siri is one of the only major differences that will convince people to upgrade from the iPhone 4. It is what it is, I suppose.