Turned on USB debugging, plugged phone into laptop, told NetworkManager to connect to T-Mobile...Done. It's amazing when you plug in a random device into Linux and right thing(tm) happens.
3G seems a lot faster on T-mobile that on AT&T. Tethering on AT&T meant things like GMail loaded painfully slowly. Latency seems high(130ms to google, 90ms more than on my Comcast, I guess HSUPA isn't kicking in), but at least it's consistent. Throughput is pretty amazing, 450K/s.