iPhone experts Steven Troughton-Smith and chpwn seem to have just gotten the iPhone 4S-only Siri working on jailbroken iPhone 4 and current-gen iPod touch units this evening. This bypasses earlier authentication issues. They tweeted their success and posted a screen shot showing Siri working via Wi-Fi (the Airplane Mode icon just means that 3G is turned off, but it is possible to turn Wi-Fi back on even in AM; that's what they did.)
Their success comes just weeks after the iPhone 4S debuted. Since Apple's back-end systems are checking for iPhone 4S devices before processing Siri queries, they managed to work around this limitation. The hack is based on moving compiled code components from a 4S to the older units.
In an interview with 9to5Mac, Troughton-Smith recounts that the Siri transplant was about a 20-step process, and that it does require access to a jailbroken iPhone 4S to work. He has no intention of releasing the mod to the public; this is a proof-of-concept only.