When i spoke to the G1 representatives (i spoke with 3 different ones) they said that the $25 plan has a certain function that the $20 plan doesnt this function is suppose to register your device to google with a gmail account. one of the reps clam it has to register through the 3G network, not the edge or gprs; and the 3G is available in the $25 and $35 plan.
but what i theorize is, if its possible to change android to wm then you can switch back to the $20 plan. otherwise, according to the 3 reps, tmobile is trying to create a plan just for the G1 for data and they said it would be priced around $20.
Wizard (custom rom)
Kaiser (custom rom)