Get Unique Device ID (UDID) under Windows Phone 8
Solution 1:
I haven't yet started to develop for Windows Phone 8, still on 7, but you still should be able to use the original DeviceExtendedProperties class to pull back the Device Unique ID.
DeviceExtendedProperties.GetValue("DeviceUniqueId")
Solution 2:
I've had this issue with returning the null value. Then remembered that it needs to be switched on.
In WMAppManifest.xml -> Capabilities tab -> switch on ID_CAP_IDENTITY_DEVICE