From previous threads at HoFo I was under the impression that T-Mobile limited their tethering to their own web and trying to tether for unlimited data violated their TOS.
I've now found this http://www.howardforums.com/showthread.php?t=1347185
which indicates it's not and you can run your own proxy server, which I've done before with HottProxy.
Looks like T-mobile may be gaining another customer.