From what I've read,
tmobile doesn't have a tethering plan for the g1, and as long as you stay under the bandwidth cap of 5gigs(???) you shouldn't have any issues.
If you go over the bandwidth cap tmobile will downgrade your data speeds to edge speeds until the next month.
Not sure how tmobile would be able to tell if you're tethering or using the phone. They could see what browser type you're using but you can change that and they'd have no way of knowing, IPtables is basically doing a NAT with the phone so your tethered device is hidden..