Siri was announced and originally made to be only supported by the new iPhone 4S, but leave it to developer Steve Troughton-Smith to port it over to the iPhone 4.
The port still needs plenty of work as the performance isn’t quite up to par and voice files aren’t being sent to Apple’s servers to be processed.
From 9to5Mac:
Developer Steven Troughton-Smith has been working with 9to5Mac exclusively to port the iPhone 4S Siri voice command system to the iPhone 4. Troughton-Smith was able to get the beginning steps of a full port rolling after installing the iPhone 4S Siri and Springboard files onto an iPhone 4. As you can see in the video, Siri’s interface loads up with all the features from the iPhone 4S implementation.
