A few months of work by iOS hackers Grant Paul (aka chpwn), Steven Troughton-Smith and Ryan Petrich has culminated in the release of Spire, a new installer that will easily set up Siri on your jailbroken iOS 5 device. While Apple has kept its little helper exclusively constrained to the iPhone 4S, it now runs on iPads, iPhone 4, iPod touches and the iPhone 3GS, and while that's been done before they claim the other new trick is that it's legal, avoiding the copyright infringement inherent in previous hacks.
When installed it will initiate a 100MB download of Siri directly from Apple so watch your bandwidth limits, and also be prepared to set up your own proxy server with donated info from an iPhone 4S to hold the line between Apple's backend and yourself. It's available on Cydia now so if you've been waiting for a way to get this feature for yourself it's not far away, however Troughton-Smith tells9to5 Mac that the next major step may not come until the iPhone 4S is jailbroken and they can avoid the proxy server issues altogether. More here.