Accessing device uniqueIdentifier is depracated in iOS7. There is Vendor ID as an identifier but it is erased with removal of the last app for that Team ID. Is there anything else used by MDM systems?
Actually, iOS MDM is purely server technology. Meaning that MDM client is integrated in iOS. What you may have seen are some MDM boostratp applications which are installed on iOS device. However, all they do are triggering device configuration (installation of MDM config profile).
However, as part of the MDM protocol, you can get UDID of device (using DeviceInformation command). However, you can get it on the server (not in your iOS app).