I have an unlocked ATT x1097 and it works just fine with tmobile.
You just have to MAKE A NEW APN SETTING.
When you put a tmobile sim in it, it will automatically create a couple of T-MOBILE apn settings. Just leave that as is and create a NEW ...
what happens if you flash unlocked AT&T LG Thrill 4G with froyo to official V21A_00.kdz ? Do I lose the unlock?
UPDATE: It worked and I see 3G appearing on my display with t-mobile
Did you flash AT&T Thrill 4G Froyo baseband to V21A?