There had been rumors that Apple was testing Siri for use on the iPhone 4, but an e-mail from an Apple rep to developer Michael Steeber has squashed that rumor. In the reply, Apple's engineering team said that "Siri only works on iPhone 4S and we currently have no plans to support older devices."
As we've seen in hacks, that doesn't mean that Siri can't work on other devices, just that Apple has no plans to support other devices. The e-mail came to Steeber in response to an idea he posed to Apple that it could sell Siri as an add-on to older devices for $19.99. It seems Apple would rather leave Siri as a selling point of the iPhone 4S though.