Some guys from mobile app company Applidium have reverse engineered the protocol used by Siri to send requests to Apple servers (e.g. a snippet of audio) and get a response (e.g. text version of said audio snippet).
It seems Apple haven't done much to prevent other parties from using this service. Though the protocol includes a device ID there is nothing in the protocol to authenticate that the client is indeed the device it identifies as so one can extract the device ID from an iPhone and use it on any other device.
I would expect Apple to update the Siri protocol to include some form of device authentication. Apple have several keys built into the iPhone they could use for this.
Updated November 17th to add:
Some of the reports on this "hack" are missing the point. For example, the Mocana blog headlines a report of this with "SSL Hack Leads To Siri Clones" and explains that this occurred because "Apple didn’t set its SSL connection correctly".
Though it is true that the Applidium researchers circumvented HTTPS encryption to analyze the protocol, the purpose of the HTTPS protocol isn't to present unauthorized access to web services, but to protect the privacy of people who use such web services. So in this case the purpose of HTTPS is to prevent attackers from gaining access to iPhone owners' data. The Applidium "hack", in which they added their own root certificate to their device, doesn't compromise this because only the iPhone owner can add such a certificate.
One might claim that Apple used HTTPS to keep the Siri protocol secret and thus prevent others from using the Siri servers. I don't think this is the case. Even if Apple had fully protected the communication with the servers, any component hacker could have easily reverse engineered the protocol from Siri's code - so I don't think Apple could have possibly relied on the secrecy of the protocol.
If Apple want to prevent unauthorized devices from using the Siri servers, they really need the protocol to include some method for the servers to authenticate the device. This shouldn't be too difficult for Apple to do as they have keys built into the iPhone for such purposes.