The mobile developers at Applidium have cracked the security controls that keep Siri working only on the iPhone 4S. The group has also released a set of tools to allow other developers to take advantage of its discoveries. Theoretically, that means access to Siri's services could be enabled on other iOS devices — or even on other platforms such as Android. The caveat here is that in order to take advantage of the tools, users will need access to a a genuine unique identifier from an iPhone 4S, its UDID.
The details of the crack involved determining exactly which protocols Siri uses to comunicate with Apple's servers as well as how Siri encodes audio — in this case, the open source Speex codec. As of this writing, nobody has used the tools to port Siri to other devices, but we imagine it's only a matter of time. It's also likely to only be a matter of time before Apple finds a way to block this method, if only by blacklisting UDIDs that are taking advantage of it.