if this is the case, here's a quick solution:
1. Download and install SGS Unlock app from Market (even if you have UNLOCK code provided by T-Mobile it's always good to double check). Please donate to developer for a great job!
2. Run SGS Unlock and write down the UNLOCK Code.
3. Turn your device OFF and remove T-Mobile SIM card.
4. Turn device ON without SIM card! Once it's on go to phone dial screen and DIAL: *7465625*638*#
it will bring you to the NETWORK LOCK screen.
- in MCC/MNC field enter code for the service provider you want to use your phone with (eg 31026 for T-Mobile USA), if you don't know a code, just GOOGLE: MCC/MNC and service provider name.
- in CONTROL KEY field enter the UNLOCK code (you got from T-Mobile and confirmed by SGS Unlock)
- hit OK
- you should get a message: LOCK SUCCESSFUL
5. Power phone down, insert SIM card you want your phone to work with and power on.
Once your phone back on you should get a message: SIM card added to SAFE list.
Now you can use your T-Mobile locked phone with another service provider.
There's no special instructions to go back to T-Mobile network. Just insert T-Mobile SIM back into your phone.
By the way, everything you do with your phone you do at your own risk! Don't blame anybody when/if something goes wrong!