I realize this thread has pretty much died, but wanted to follow up on my previous post regarding MMS no longer working after a few days using the default APN and had to manually the "phone" APN to get things working again.
Basically, things had worked fine for about the first week using the default "nxtgenphone" APN, then MMS started having problems, added "phone" APN and worked fine again for a few more days, then MMS completely stopped working, restore, re-flashed, etc still didn't work. Finally, reverted back to MJ5, still didn't work, then went ahead with the NB4 update... NO Go... this was all after using the leak for approx. 2 weeks.
Finally called AT&T and actually received pretty good support; immediately escalated to a Sr. Support Tech.. long story short, he determined that my phone had been put into a "holding" state preventing it from properly registering with the tower in my area. He explained that currently the "nxtgenphone" APN is not approved in all locations and has seen this occur with other phones that haven't had their tower updated to use this particular APN.
He was able to resolve the problem by removing my phone from the tower, re-registering and all is working now.... your mileage may vary depending on location, but wanted to pass on in case anyone else was experiencing similar problems.