Activating and upgrading HTC G1 w/o a data plan or a sim?
Most updates won't erase your data. The kernel and /system partition can usually be updated without the filesystem needing to be wiped, so your data folders aren't damaged.  If the WiFi network data isn't erased, then your phone should connect to WiFi after the upgrade, and you're good to go. I can't find any info on what happens when upgrading to RC33, for example, but official updates usually don't wipe data. The update might also eliminate the No-SIM-so-you-can't-do-anything problem.