Siri is no longer the monopoly of iPhone 4S. Now you can use Siri on all iOS Devices and with a new app called Spire.
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 can mirror all the iPhone 4S Siri functionality. Spire can make Siri work on iPhone 4, iPhone 3GS, iPod Touch 3G, iPod Touch 4G and iPad (first generation).
In his blog post, Chpwn said that the size of Spire is around 100 MB. The developers recommend connecting Spire to a Wi-Fi network (instead of a standard cellular network) network because of size of Spire as a file is large. At the moment, there have been a few Siri ports which have gone public through Cydia. However, they were all deemed to be unsafe and illegal by the development community. This was a result of these ports distributing Apple copyrighted materials. Chpwn says that the Spire uses a new method to obtain the files necessary for Siri. That is why, it does not have the copyright issues encountered by previous attempts.
However, you would still need to gain authorization through your own server and an iPhone 4S. Chpwn elaborated:
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.
Steven Troughton-Smith said that the proxy server issues can be avoided when iPhone 4S is jailbroken.