Hacker/Developer Steve Troughton-Smith has managed to get Apple's Siri voice interface working on an iPhone 4 and iPod Touch, even tricking Apple's servers into supporting it against policy.
In an interview with 9to5Mac Troughton-Smith describes the work and provides the video embedded below.
Troughton-Smith had ported Siri quickly and had it working almost as Apple released the final iOS 5, but to work properly it needs to communicate with Apple's servers. This work he has finished in conjunction with hacker chpwn.
As chpwn explains in his blog, they have not released this hack to the public because it relies on an as-yet unreleased iPhone 4S jailbreak. He also notes that the hack requires that the user own an iPhone 4S in order to comply (at least arguably) with Apple's copyrights because it requires copying Apple files from the 4S to the earlier device.
Apple disappointed many users when they released iOS 5 for the iPhone 4, but made Siri available only on the 4S. The hardware in the 4S is more powerful than the 4, but Troughton-Smith and chpwn's hack shows that it can run. chpwn insists that their hack is the only one that properly communicates with Apple's servers.