Although T-Mobile is working on a firmware update for the G1 phone, there’s a hack that lets you update to the latest firmware over at MobileCrunch. There’s a note that this hack isn’t really supported by T-Mobile so if you are going to do it, just make sure to follow directions carefully!
Here’s the directions:
1. Grab the RC29 update from Google here.
2. Rename it from signed-kila-ota-115247-prereq.TC4-RC19+RC28.zip to update.zip, and place it in the root of your Micro SD card.
2.5. If you added the file to your microSD card by connecting the handset to your computer via USB (rather than using a microSD card reader), make sure to unplug it now. (Thanks for the tip, Cole!)
3. Turn off your G1, then turn it back on by holding the Home+End keys until you see an icon popup after the T-Mobile G1 logo.
4. From the icon screen, open the keyboard and hit Alt+L. This will display the log saying what is happening (although you can skip this step).
5. Now press Alt+S to begin the update. Remember, the update needs to be in the root of the Micro SD card and needs to be named update.zip.
6. Once it finishes, just follow the onscreen instructions and press Home+Back to finish.
7. The G1 will reboot a few times to flash different parts of the firmware.
If all went well, you should now be running the latest update, RC29. To make sure all your hard work wasn’t for nothing, hit Menu > Settings > About Phone > scroll to Build Number. You should see RC29 in there somewhere. Congrats!