Following Mapplegate, Apple could be dealing with Sirigate.
Heise.de reports that Siri may not work if a user changes the type of network connection. Switching from Wi-Fi to 3G/4G or switching between LTE and 3G allowed the staff to reproduce errors and prompt the feature to hang. According to Heise, both iOS5 and iOS6 are affected, and Siri is the only feature on the iPhone that becomes unresponsive when the network connection is changed. Only a reboot of the iPhone can bring Siri back to life.
Apple did not comment on the report and did not confirm that there may be a problem that impacts the functionality of Siri.