Previously, T-Mobile didn't officially support tethering, but neither did they restrict users from tethering their cell phones (in fact, I remember tethering a T-Mobile cell phone to various PDAs via infrared back in the '90s). Since November 2010, however, T-Mobile has been officially supporting tethering -- and charging for it. The phone tethering and wi-fi sharing plan runs you $14.99/month, on the low side of tethering charges among the major wireless carriers in the U.S., but still an extra charge that doesn't give you additional data usage.
How To Tether Your T-Mobile Cell Phone
T-Mobile instructs users to refer to their user forums to configure their phones as modems. The instructions reinforce the need for a data plan on your cell phone and link to phone-specific (BlackBerry, Windows Mobile, Android, and Nokia) setup instructions.
An easy and universal way to set up tethering on your device, though, is to use an app like PdaNet, because you don't really need to change detailed settings. For more phone tweaking, the community at HowardForums is a fantastic resource as well.