I have your answer, at least for Windows 7.
Get some drivers. Covered elsewhere. If you get the Kies software, it provides drivers.
Connect the phone to the PC using the USB data cable. USB connection must be in kies mode.
Windows will automatically detect the internal modem of your phone
Go to the control panel and select "Phone and Modem" (Windows 7 - you'll have to figure out for others).
Click on the "Modem" tab.
Select the modem by highlighting it.
Select "Properties".
In Windows 7, at the "General" tab, select "Change Settings" - this will ask you to allow it to change the system. You must say yes,
for the rest to work.
Select the "Advanced" tab.
Type in:
at+cgdcont=1,"IP","internet2.voicestream.com"
Do this EXACTLY as above, including double quotes.
Select the "Diagnostics" tab.
Select "Query Modem".
Click "OK" after seeing the word "Success" in the response.
You should be able to use ANY 3G phone provided by T-Mobile that has a CDMA internal modem to connect your tethered PC to the internet.
What this does is send some configuration information to the phone, which it saves, and which tells it further information on how to connect when tethering. I don't know whether this is retained when the phone reboots, but it probably is. I'd suspect you will need to repeat this if you flash update the phone.
[SETUP DIALUP CONNECTION]
For Windows 7:
1. open start menu, type "dial up" - wait for it to find "Set up a dial-up connection" - go there.
2. When it shows list of modems, select your phone - mine says "SAMSUNG Android USB modem"
3. Enter ONLY the phone number: *99# (that's star, 99, pound)
4. Change the connection name to whatever you like, such as "T-Mobile".
5. Click "Connect"
It should connect. If not, possibly some step above isn't correct. Of course, it is possible T-Mobile did something since I wrote this, to disable it, but it worked fine for me after having this exact problem.
as phone number. All else unchanged (but can set name of connection)