The T-Mobile G1 hasn't even been out a full week and its already been unlocked to work on AT&T's network.
"Unlocking the G1 is very quick and easy, in fact anyone who can make a phone call can unlock their G1 with no trouble at all. Unlock-TMobileG1.com offers unlock codes for $22.99 USD. After placing an order including your IMEI code you will receive an eight-digit unlock code for your device. Your IMEI number can be found by pressing *#06# in the dialer, or by going into your settings, selecting “About phone” and then selecting “Status”. The IMEI number then can be found about half way down, it is 15 digits long." Via Android Community .