Tethering on T-Mobile is free. I don't have cable/DSL at home. Rather, I tether through my T-Mobile cell phone exclusively. Word on the street (T-Mobile forums) is that there is a 5 GB soft cap on data. Once you hit it, you are dropped to Edge speeds.

Last month I went over 5 GB for the first time (5.5 GB) but speeds were not dropped.