Siri has been the most discussed feature of iPhone 4S and iOS 5 and since its inception hackers community are working hard to port it to older Apple devices including iPhone 4 and iPod Touch. However this time its not about port rather its about making custom Siri commands by learning in the new way.
Aman Gupta, an iPhone developer has been working on custom Siri commands and through Chpwn he has confirmed that developers can add custom commands in Siri which are shown in Screenshot below,
Custom commands open new horizon as you can make Siri learn what you want and respond to specific actions. Although you do not require the device to be Jailbroken but you require network proxy which can let you redirect Siri traffic through custom proxies allowing manipulation in Siri command structure. However in final release you might see it working through Jailbreak as custom mods need to be inserted before transmitting specialized commands. However there is no chance that it requires Siri Software legal and piracy issues as Siri Port.