A trio of iOS developers: Grant Paul (Chpwn), Ryan Petrich, and Steven Troughton-Smith have recently released a new Siri port called Spire. This port mirrors all the iPhone 4S Siri functionality and works on the following iDevices: iPhone 4, iPhone 3GS, iPod Touch 3G, iPod Touch 4G, and iPad (first generation).
According to a blog post from Chpwn, Spire comes in at around 100mb and the developers recommend connecting to a Wi-Fi network for downloading, not a standard cellular network because of its large file size. As of right now, there have been a few Siri ports which have gone public through Cydia but they were all deemed to be unsafe and illegal by the development community. This was a result of these ports distributing Apple copyrighted materials. According to Chpwn, “Spire uses a new method to obtain the files necessary for Siri, so it doesn’t have the copyright issues encountered by previous attempts.”
There still is a big caveat though – you still have to gain authorization through your own server and an iPhone 4S. Chpwn took the time to explain the caveat with a bit more detail:
However, Spire is not a complete solution. Apple still requires authorization to use Siri, so information from an iPhone 4S is still required. To insert this information, Spire allows you to enter your own proxy server address. By using this (ancient) SiriProxy fork, you can setup a proxy using your own iPhone 4S to insert the needed information reasonably easily. Other solutions for proxying Siri will be listed here as they are developed — perhaps that sort of proxy might be included in the main SiriProxy repository.
Source: Chpwn’s Blog (1) (2), Steven Troughton-Smith (Twitter)