A lot of us (me included) mindlessly upgraded to the RC30 firmware on our T-Mobile G1 and immediately realized that we could no longer have root access. Fear not, chavonbravo, an astute member of the XDA-developers forum got their hands on the RC29 firmware and uploaded it for all to use.
Taking a look at the directions, it doesn't look especially tough to flash back. If you want to gain root access again, check out the step-by-step instructions from the XDA after the break!
If you have RC30/RC8 or later, you must FIRST flash your phone to RC29/RC7 or lower to be able to exploit root access using the following steps. RC29 and lower users can skip this section. 1. Format the SD card to FAT32 mode (this is sometimes needed. The image is not always found if other files are on the sd card). 2. Unzip the RC29 or RC7 image file DREAMIMG.nbh to the SD card. (RC29 for US, RC7 is for UK) 3. Turn the device power off and insert SD card. 4. Hold Camera button, and press Power button to entry bootloader mode. 5. Press Power button to start upgrade procedure. 6. After finish, perform the soft reset to reboot. Follow these steps once you have RC29 or lower: On your G1, anything you type into your keyboard is also being run in a hidden console with root permissions. More information regarding that at the bottom of this post. But, to get root access, do the following: Instructions: 1. Restart your phone 2. Type telnetd and press enter - Yes, it will start up a Contact search, do not worry about this, just type telnetd and press enter (the enter button on the keypad). 3. Download an Android Telnet client (I have one on the Market) and connect to localhost. 4. you now have root!