Someone has discovered a way to get Siri dictation working on an iPhone 4 (other iOS 5 devices should work, according to the video, but they are using it on an iPhone 4.) The jailbreak app is called Siri0us (the ‘o’ is actually a zero).
You will need a jailbroken device on iOS 5. You can check out our guide here, to find the tools you need to jailbreak your iPhone. Here’s the Cydia repository where you can pick it up. As for Siri’s use as a personal assistant, like what’s used on the iPhone 4S TV commercials, developers are still working to make it work.