How to register and sign in the Tmobile G1 without data plan on tzones hack
The key to sign in to your G1 is to have internet connection. If you have a g1 data plan, you connect through tmobile’s epc server and sign in. You should have the g1 data plan when you first get your G1 from tmobile, but for some reason if you reformatted your phone or downgraded/upgraded and needed to sign in again, you can do it through tzones or by enabling the wifi.
To do this without tzones through wifi, you need to have root access on your G1. check this forum for help with turning your wifi on.
If you have tzones, hit the menu button when it asks you to sign in. click on apn settings and change the Tmobile US settings by following my older post. Now you should have internet access through WCDMA and you should be able to sign in.