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, Follow these basic steps. I use Ubuntu Linux, but you can find directions on the web for Windows or Mac:
1. Connect your phone to your computer so you can use adb. Check this post.
2. Open up terminal and type:
am start -a android.intent.action.MAIN -n com.android.settings/.Settings
This will put you in the settings and will allow you to connect to wifi. For more help you can check the xda-developers forum.
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.