AT&T blacklisting only works on their own network and will not affect other gsm networks like tmobile and their subsidiaries. It is not like in Europe where carriers will share a blacklist. Tmobile I don't believe use a blacklist system of any kind.
thanks. shouldve been more careful but **** happens. even though the phone is gone, it's good to know that the scumbag will not be able to activate it on any GSM network within the states