The Next Web reports that app developer Applidium, who you may remember from the VLC iOS app fracas earlier this year, has cracked the security protocols associated with Siri. Using Applidium's methods allows Siri to recognize voice commands from any device -- not just another iPhone, but also an iPad, or even potentially a Mac or Android device.
As of now the only thing keeping this from reaching a widespread audience is the fact that an iPhone 4S UDID is still required to get Siri to recognize inputs. Technically an enterprising developer with the proper know-how could tweak things to associate the iPhone 4S UDID with another device (as Applidium has done), but as The Next Web points out it would be very easy for Apple to track down such activity and shut it down.
Applidium has a post describing its methods as well as some of its other findings. Among the interesting things they learned: Siri sends compressed audio in Speex format (an audio codec associated with VoIP) of your every command to Apple's servers. Other data includes timestamps and confidence scores for each word spoken, all of which presumably helps out a great deal in deciphering the actual content of your commands. In other words, our assessment from a few weeks back appears to be correct: the device itself determines what you said, while Apple's servers take that data and extrapolate it into what you meant.
Applidium's work is just the latest bit of evidence that there shouldn't be any technical hurdles holding Siri back from more widespread deployment than just Apple's newest iPhone. Despite Apple engineers' comments that they "currently" have no plans to spread Siri beyond the iPhone 4S, it seems inevitable that the service will eventually expand to other devices in Apple's ecosystem.