Siri0us – is free on the Cydia store and provides Siri dictation on iPhone 4, 3GS. We won’t see a full port anytime soon coz there are couple of legal issues, thanks to Apple’s control freaks.
Siri dictation works on all iOS5 devices. To install Siri0us, add the http://apt.if0rce.com repository to Cydia and search for the app “Siri0us”.
Note: In case you encounter continuos resprings, use a SSH tool to remove the /System/Library/PrivateFrameworks/AssistantServices.framework file.
The app works great on all iOS 5 devices, without any trouble.
Here’s Siri0us on iPhone 4:
Update: Spire: Install Siri on iPad, iPod Touch, iPhone 4, iPhone 3GS