Barjohn - Congrats on getting this unlocked and working on ATT.
However, as I mentioned in a post on HOFO according to my conversations with RIM, this will only allow the phone portion of the 7100t to be used on networks other than T-Mobile. According to RIM other networks such as ATT or Cingular are not setup to provide BB service to the 7100t. So in other words an unlocked 7100t will work fine as far as receiving/making calls but it will not operate on the network as far as getting mail pushed to it.
I have talked to RIM twice about this and each time they have confirmed this to be true. If anyone out there has been able to accomplish this setup I would love to hear about it.