Our G1 was acting flaky--frequent reboots, as if the battery were disconnecting--so we got a warranty replacement. We still don't have a data plan, so activating the G1 is still interesting. My old hack to activate the G1 without an Android data plan (T-Mobile Android Unlimited Web) doesn't work any more; it looks like T-Mobile disallows G1 requests through the old WAP gateway.
Fortunately, wife just got a myTouch 3G and the matching Android data plan. To activate the new G1, we simply put wife's SIM in the G1, boot it, and activate the G1. We put the no-data SIM back in the G1, connect to the local WiFi network, and we're all set.