This is my first post. I have been using an alternate way that I figured out to return to stock. This also works when you try to install an official update and it rejects the package due to invalid keys or device checks. You MUST already have root on your phone for this to work.
1) download official firmware image (whatever version you want)
2) extract recovery image from package to root of SD card
3) rename downloaded package to update.zip and place in root of SD card
3) reflash recovery image from terminal on rooted g1
mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system
cd sdcard flash_image recovery recovery.img
cat recovery.img > /system/recovery.img
4) Restart phone in recovery mode and proceed with update
5)Profit! Well, stock g1 without having to flash to RC29 and wait for an update.
I feel this is a more straightfoward way to flash back to stock. YMMV.