iOS (Jailbroken): Siri, the iPhone 4S voice assistant/voice dictation system, has been locked out of other iOS 5 devices like the iPhone 4 and 3GS by Apple. Hackers have gotten it to work on the older phones in a proof-of-concept way, but nothing has been released that makes things work as well as the iPhone 4S Siri. But, if you want the voice dictation aspect, there's Siri0us.
First you'll have to jailbreak your phone, but when you're done, go to Cydia and add this to your repository by clicking Manage -> Sources -> Edit -> Add:
http://apt.if0rce.com
Then, whenever you're typing something, hit the microphone button (just like with Siri) to start dictating to your phone. It'll hit up a web service, so you need internet access, and send back the dictated text. Siri0us will also work on the iPod touch 3G and 4G.
Like all Cydia apps, the onus of compatibility is on you. In this case, there's a possibility of Siri0us making your device reboot unexpectedly, but you can take care of that by removing a file via SSH manually. They explain how to do it here.
Add Siri Voice Dictation To iPhone 4, 3GS & iPod Touch With Siri0us [Cydia Tweak] | Addictive Tips