System wide voice-to-text dictation, once exclusive to iPhone 4S, has now been replicated on other iOS devices thanks to a Jailbreak hack called Siri0us. In order to use the dictation hack, you will need to have a jailbroken iOS 5 device. A point to note, the hack does not use Apple's servers, it solely uses the Nuance speech recognition system on device, and it does not provide any of the Siri voice control features.
Currently it has been confirmed to work on an iPhone 4, iPhone 3GS and iPod touch 4 generation. As we mentioned, the hack uses Nuance Speech Recognition however it is not officially licensed. The developer is hoping that Nuance will offer him the ability to purchase licenses to make the hack legal.
If you want to give it a try, add http://apt.if0rce.com to your list of Cydia sources, you should then be able to search for and install Siri0us. There appears to be a huge demand for the hack as the repository holding the app is getting hit with a lot of traffic and keeps going down.