[SmoothOperator]

I'm wondering if there will be network compatability issues with AT&T when using an unlocked 9780 branded for another company, say T-Mobile. Is anyone here presently doing that without problems?

[Matrix Leader]

You need to check what bands the AT&T operate on and what bands does the T-Mobile operate on, if they are the same, then it'd work. If not, then you're out of luck.

[NJBlackBerry]

The OP asked specifically "Is anyone here presently doing that without problems?"

Since many of us already know that you will not get 3G on AT&T with a T-Mobile branded device, the answer is "no". Since I doubt very many people have tried it yet, no one here has answered "Is anyone here presently doing that without problems?"

AT&T and T-Mobile US use different 3G bands.

[JSanders]

...which means you will get "EDGE" and of course speedy data transfer when connected to WiFi.

The user needs to decide if they can live without 3G speed when away from WiFi.