@Andor. You are completely wrong. in all aspects
1. Updates are pushed out from google not from t-mobile.
2. Over The Air (OTA) updated will also work in non t-mobile networks, such as Vodaphone, O2, etc. In all countrys where G1 is official available
3. You don't need root at all to manually update:
3a. If the update file is available, rename it to update.zip
3b. save update.zip to root of sd card
3c. turn off phone
3d. restart phone by pressing On/off button and home button until yellow triangle appears
3e.press ALT+S
3f. wait....
3g. ready