While Apple has limited Siri usage to just the iPhone 4S, iOS hackers Grant Paul (aka chpwn), Steven Troughton-Smith and Ryan Petrich have initiated a download of Siri using Spire Installer on any Jailbroken iPads, iPhone 4, iPod touches and the iPhone 3GS considering that the device runs on iOS 5 (Interesting since an untethered jailbreak for iOS 5.0.1 just came out). The Installer legally downloads a 100MB Siri installation directly from Apple and this is all legal mind you.

The Siri port to run however requires a proxy iPhone 4S with instructions available on the link below. The team reports that once iPhone 4S is jailbroken the need for a proxy device may be eliminated altogether.

[chpwn blog]