Enabling Siri on the original iPad or the iPhone 4 requires your device to be jailbroken, and despite the fact that Siri will load on these devices, it won’t actually retrieve any results, as Siri relies on Apple’s servers to function, and only the iPhone 4S is able to communicate with those servers.
ModMyi is maintaining a FAQ on porting Siri to other devices in order to keep those interested informed of their progress.