They updated their Terms of Service last June to explicitly disallow tethering. They also has the original android-wifi-tether app removed from the market when it was first released (although Google only removed it for T-Mobile customers). Not sure why the haven't also banned PDAnet and EasyTether, probably because neither does wifi tethering so they don't require rooting. T-Mobile have sold phones that can tether for a long time. My 3 year old Moto Rizr could do it via USB or Bluetooth, also the Nokia 6133 and the Razr before it. Even though it's expressly verboten I've never heard of anyone receiving a smackdown from T-Mobile for tethering.
There's plenty of posts on multiple forums of people trying to tether on Verizon and AT&T and instead being informed that they have to buy a data plan, just as there are people on both networks who are tethering care free.
So it's pretty much a try-it-and-see At Your Own Risk proposition.