It’s a bit of a misnomer to say that this is the full unlocking process, but this procedure will let you go through most of the T-Mobile G1’s features without actually activating a SIM card from T-Mobile.
If you’re not quite prepared to sign a two-year contract, you can actually use the T-Mobile G1 on Wi-Fi alone. It’s a simple five-step process. First, you’ll need to borrow an activated T-Mobile SIM card from a friend. Second, go through the setup process to link to your Google account. Third, enable Wi-Fi in the settings. Fourth, connect to a nearby hotspot. Last, remove the SIM and insert the unactivated SIM that came with the G1.
Tada! A T-Mobile G1 that does only Wi-Fi. I have a feeling that this process won’t let you insert an AT&T SIM card and latch onto that GSM network instead, but if you happen to find out to the contrary, do let me know!