Following weeks of efforts reverse-engineering Apple's Siri personal assistant feature, a trio of developers have come up with a solution that's said to work on any jailbroken device running iOS 5 and avoids the copyright infringement inherent in previous hacks. Dubbed Spire, the new Siri port is available for free on Cydia and was tested to work on the iPhone 4 and 3GS, third- and fourth-generation iPod touch, and first generation iPad.
Spire is the work of well-know iOS hacker chpwn (aka Grant Paul) along with developer Steven Troughton-Smith and Ryan Petrich.
Unlike previous Siri ports, Spire obtains the files necessary for Siri from Apple's own servers, so technically this is a "legal" hack since the files are not being widely distributed by a third party. However, because Apple only officially supports Siri requests coming from the iPhone 4S, you'll still need access to a 4S' authorization token and must route that information through a proxy server.
Chpwn explains the limitation in the Spire Proxy FAQ, "Apple has made it very likely impossible to defeat the authorization requirement. I reverse engineered it, and it does not appear possible to connect Siri to the cloud without information from an iPhone 4S." He goes on to suggest you could ask and iPhone 4S-owning friend for their authentication tokens, or perhaps someone will come up with a for-pay service where you can rent some space on a Siri proxy attached to one of their iPhone 4S devices -- though if you really want Siri that bad maybe you should consider getting an iPhone 4S and avoid jumping though hoops.
At least until the iPhone 4S is jailbroken and hackers find a way around the proxy server issue, this is the best Siri port so far. That said, if you already own an iPhone 4S and just want Siri on another device of yours you can download the 100MB app and give this a hack try.
Remember you'll need a jailbroken device to install Spire. Conveniently enough, the iPhone Dev Team just released an untethered jailbreak for devices running iOS 5.0.1 (not including the iPhone 4S and iPad 2).