Something to try that worked for me for the time being
I just thought I would share with anyone else having issues with the persistent Preparing Network message.
If you go to Settings>Activate Device and launch the Self Service application, there is a hidden menu accessible via the Menu Key, And you can then choose Reset Cellular Data.
Your device will then reboot, and it will go through Self Provisioning when it reboots.
I Had to do this twice, and 2nd time seems to have been the charm this time as the Preparing Network Message is gone!