It's Simple to unlock the T-Mobile G1,You can use the Unlock code to Unlock T-Mobile G1,But free unlock code is not possible to get,You need to pay for it.Buy the Unlock code from here
Theunlockspot.com and unlock it from Network Lock.Because T-Mobile G1 is latest Phone,You need to pay for it,You can get the free Code for Nokia DCT Phones .