iPhone Dev-Team member MuscleNerd has gone on the record stating a port of Siri to other iOS devices outside of the new iPhone 4S will require piracy, and likely won't ever see the light of day as a consequence.

Anyone hoping for a "port" of Siri from iPhone4S: pending a very low-level A5 exploit, it likely can't be done without piracy

(only way to extract Siri files from iPhone4S filesystem without pirating is to have the main FS keys, requiring low-level exploit)

Essentially, the only way to get Siri working on other iOS devices would be to take the required files from an iPhone 4S and transfer them onto the other device manually. It's also possible that Apple may flag such devices since Siri talks to Apple's servers to complete certain tasks.

On the other hand, piracy in the jailbreak community has been running rampant lately, so it's also possible we could see this come from other, less ethical jailbreak developers.

Source: MuscleNerd