From Android Wiki
First off, be advised if this process fails, you have a very pretty looking paperweight and from what I have heard, your dog might die. Your device will not recovery from this if you mess up. That said, HAPPY FLASHING!!!
- Download the engineering bootloader
- Rename it to update.zip and save it to /sdcard on your G1
- Enter the recovery console (like you did when you flashed JF's modified update) by performing the following steps:
- Hold down power and home to enter recovery mode
- At the exclamation point and phone image hit Alt-L look for test-keys on the second line. If missing, you are either on JF's 1.2 (he forgot to put the "using test keys" in that version) and are fine or you have not installed the modified recovery like you were told to and should stab yourself in the face now.
- Press Alt+S. The system will proceed to verify the image and install it. When done, it will say to reboot by pressing Home + Back.
Confirm your phone has the engineering bootloader by turning the phone completely off, then rebooting holding the power + back button. If you see skateboards, you win.