Siri was legally ported to iPhone 4, iPod Touch 4G and iPhone 3gs. As you may know that from release date of Siri (Apple’s voice activated personal assistant available for iPhone 4S) hackers were trying to port Siri to other iDevices i.e. other than iphone4s.
You can watch this video where Siri was ported to 3gs.
Now you can port Siri to your iphone4, 3gs and ipod touch 4g by using a tool called “Spire” by Grant Paul, Steven Troughton-Smith and Ryan Petrich. Only what you need is to jailbreak your device before porting Siri to your iOS 5 jaibroken devices including iPhone 4, 3GS, iPod touch and iPad.
For more details I suggest to visit Cydia and search for Spire. Also it is recommended to visit chpwn blog and Spire Proxy FAQ blog.