UDID not available on iOS 6
Since iOS 6, apps are no longer allowed to access the UDID (unique device identifier) for privacy reasons (some developers used the unique identifier to track users). For this reason, System Status can no longer show the UDID.
However, if you need to know your UDID, there are still ways to access it — either using a special profile or using iTunes. For more, see, for instance, here: