You are probably going to be cheaper buying a new one on ebay, Bongowireless (
http://www.bongowireless.com/pages.php?pageid=16#11) will unlock if for $100, but then you will still have the ATT software in it...
I've seen almost new 7230's on HoFo for as low as $180.
If you were a T-mobile customer they would provide the unlock code, but only on their own devices.