OK, here's the scoop (I am writing this via tethering with my MT4G).
Firstly, if you own a Mac, forget it; Macs are not yet supported (luckily I have a PC as well).
You must have tethering/Hotspot plan ($15 with T-Mobile)
Quick steps:
A. The driver for USB tethering is stored on the memory card. You need to install this driver on the computer before you can share the phone’s mobile Internet with the computer over a USB cable.
To install the USB tethering driver on the computer, follow these steps:
1. Connect the USB cable to the phone and a USB port on the computer.
2. On the phone, touch and drag the Notification bar down.
3. If it is not set to Disk drive, tap Select to change USB connection type.
4. Tap Disk drive.
5. Tap Done.
6. On the computer, wait for a Removable Disk window to appear on the screen.
7. Double-click Open folder to view files using Windows Explorer.
8. Locate the HTCdriver.exe file on the memory card.
9. Double-click the HTCdriver.exe file.
10. Follow the on-screen instructions to install the driver.
11. Disconnect the USB cable from the computer.
12. Reconnect the USB cable to the computer.
13. Touch and drag the Notification bar down.
14. Tap Select to change USB connection type.
15. Tap USB tethering.
16. Tap Done.
B.
To turn on and set up Portable Wi-Fi hotspot, follow these steps:
Using hotspot app
From the Home screen, touch the App Drawer.
Touch Wi-Fi Hotspot.
Under Settings, set the following options:
Router name (SSID)
Security
Password (8-63 ASCII characters)
Under Portable Wi-Fi Hotspot, touch Portable Wi-Fi Hotspot to select the check box.
Wait as the hotspot activates and the Hotspot icon appears in Status Bar.
Read the instructions, and touch OK.
On the device you want to connect to the hotspot, set up the Wi-Fi connection. Use the router name (SSID), security, and password you set previously. Your Wi-Fi devices can not connect to the Internet whenever you have a cellular data network.
Note: Please read the disclaimer and notes on unsupported handsets, devices, and software.
Using settings
From the Home screen, press the Menu key.
Touch Settings.
Touch Wireless & networks.
Touch the Portable Wi-Fi hotspot check box to select it.
Wait as the hotspot activates and the Hotspot icon appears in Status Bar.
To change settings, touch Portable Wi-Fi hotspot settings.
On the device you want to connect to the hotspot, set up the Wi-Fi connection. Use the router name (SSID), security, and password you set previously. Your Wi-Fi devices can not connect to the Internet whenever you have a cellular data network.