Developer Steve Troughton-Smith and the 9 to 5 Mac community have managed to take the Siri voice recognition software that is one of the selling points for the Apple iPhone 4S, and port it over to the single-core iPhone 4.
So far the hack has numerous flaws, with the main ones being that it can't use Apple's server-side processing, and the GPU acceleration that makes it actually seamless, thus it mainly serves as a proof of concept of sorts. It recognizes spoken commands in the usual Siri interface, and also works in dictation mode, with both being rather sluggish.
The silver lining is that Siri doesn't seem to be tied with the A5 hardware, as most rumors before the iPhone 4S announcement were pointing out, but hardware-accelerating it on the iPhone 4 and connecting it to Apple's servers are still a pipe dream for now. Have a look at how the Siri port behaves on the iPhone 4 in the video below.