What fortuitous timing! Intending to start from a clean slate, I performed a factory reset on my G1 recently. I had forgotten that, unlike the Developer Edition of the G1, even an unlocked standard issue G1 requires a SIM card with a data plan for the initial setup.
I was going to wait until I could borrow a suitable SIM from a friend for a few seconds (as after registration, I'm happy to use just WiFi for my G1 net needs for the time being), but I just heard about a new jailbreak for all G1s. It works by flashing the G1 back to RC29 where the old jailbreak can be exploited. Once done, one can register the G1 via WiFi, without a data plan or even a SIM.
Some Linux-specific notes. The G1 is touchy about the SD card for the RC29 image. The time it worked for me, I had reformatted it with "mkfs -t vfat /dev/foo", and used uppercase exclusively for the filename. To get the "adb" tool from the Android SDK working, I had to create an appropriate /etc configuration file.
I only needed two of the incantations. On the G1, I enabled adb via:
<enter>setprop persist.service.adb.enable 1<enter>
and once in the adb shell, I opened the Settings menu before registration:
$ am start -a android.intent.action.MAIN -n com.android.settings/.Settings
From here it was trivial to setup WiFi and sign in.
Some unrelated tips. When typing, Alt + Delete clears the whole line. RemoteDroid turns the G1 into a wireless keyboard and mouse; a fun application to show off.
I remain easily amused by the digital compass. The Orienteer app is often the first app I demonstrate. Also, the Sky Map app is perfect for those interested in picking out constellations in the night sky, but not interested enough to actually memorize them and figure out when and where they can be seen.