Also, the tethering app is awesome. It turns your G1 into a WiFi base station and routes traffic from WiFi to 3G. Since I'm still waiting for broadband at my new apartment, it's a lifesaver.
I suppose tethering (and rooting the phone) technically violates T-Mobile's TOS, but I'm convinced that T-Mobile will allow both sooner or later. It's just too awsome, and it would help them sell more contracts.
It's kind of difficult to abuse tethering anyway; it sucks down the battery very quickly, and the latency is significant. It's the sort of thing you'd only use in a pinch. Those happen to be the situations where a little benevolence or selfishness from a big company can shape a customer's opinion forever. T-Mobile seems to be more sensitive to that kind of thing than the other networks. I know they've got their reasons for banning tethering apps, but I think they could be convinced to change their minds. (You can download various petitions from the Android Marketplace.)
Openness is where Google and T-Mobile could really go after the unwholesome, anticompetitive and un-American AT&T/iPhone alliance. The open nature of Android is a step in the right direction, but T-Mobile needs to get its legal department on the Open Access bandwagon if it wants to press the advantage.
After all, if some random people on the internet can roll better firmware for the G1 than their in-house developers, isn't it a strategic business advantage to let them?