iPhone jailbreak developer chpwn, along with the help of planetbeing and Ryan Petrich, have released Spire -- a legal way to port Siri to currently unsupported iOS devices like iPhone 4 and iPad 2.
Spire uses a new method to obtain the files necessary for Siri, so it doesn’t have the copyright issues encountered by previous attempts. Similarly, rather than directing all traffic through a specific proxy server (and the associated privacy issues), Spire allows you to specify your own proxy server.
Available in Cydia now, and if you need extra help, jump into our jailbreak app forum.
Source: chbwn blog