http://namshik.com/how-to-tether-your-t-mobile-htc-g1/
That being said, I want to make a disclaimer:
I am not sure of the legality of tethering. Apparently, from what I found online, T-Mobile has a particular plan which allows you to legally tether your phone, at around $30 a month. If you are not using their plan, DO NOT tether in the first place: it's a violation of the terms of service for the phone, it voids any protections to software that may be damaged as a result of the tethering, and you can fairly easily be caught for it since the internet usage limits allowed by T-Mobile's plan allow them to distinguish between legal and illegal use of the software.
Note: Even in this area, I'm not 100% sure to the accuracy of my comments. If someone wishes to clarify the legality of tethering, please let us know.
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