I'm not sure how it's possible, but apparently @Jackoplane has gotten Siri Running Successfully On All iOS 5 Devices,
No Jailbreak. Not sure how you'd run it (as a normal app maybe?) and if so does it hook in and integrate the same as Siri on the 4S (no idea how).
Still, I'm excited for this, especially in the "is it restricted to the 4S because of marketing or technical reasons" question.
Update: Jack says you do need to jailbreak to get it to work.