We know that Siri works on the iPhone 4 as shown by a previous hack but it required files from the iPhone 4S to work. That hack showed that Siri does not really require iPhone 4S-level hardware for it to work.
Now a jailbreak called siri0us brings Siri-like dictation feature to the iPhone 4, iPhone 3GS and iPod Touch 4G. Siri0us is now publicly available in Cydia and it’s proven to work. Developer Eric Day describes the jailbreak as “Siri dictation for your iOS 5 devices. No iPhone 4S keys/files required.” The technology is Naunce-based dictation but it is said to imitate Siri well.
Watch this video if you want to see Siri0us in action.
If you want siri0us on your device, add http://apt.if0rce.com to your list of Cydia sources and search for “siri0us.” This requires a jailbroken device.