After getting multiple looks at the Siri port on unsupported iOS 5 devices, Steven Troughton-Smith, Chpwn, and Ryan Petrich have finally publicly released a Siri port called Spire. The port brings all the Siri functionality to most jailbroken devices.
This includes: iPhone 4, iPhone 4GS, third-fourth generator iPod touch, and first generation iPad. Spire is available for free on Cydia and weights in at about 100mb. A wi-fi connection for download is recommended.
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.
What makes Spire the best Siri port out there at the moment, as pointed out by Chpwn, is that it uses a new method to obtain the files needed for Siri, so it’s completely legal and doesn’t have any copyright issues.
If you just picked up a new iOS device this holiday and are already jailbroken, you may want to check this out for pure Siri awesomeness. If you have any other question as to how Spire operates, click this FAQ.