Originally Posted by wmm
Now we just need to wait and see what your next bill looks like.
Since it is a carrier-specific phone, even with a mostly-vanilla Android build on it, I wouldn't be too surprised if the tethering app had been modified to notify T-Mobile that you were using it.
Thankfully I didn't do too much on it. Just connected the N1 to see if it would work and then turn it back off. If push comes to shove I'll enable it only when I'm travelling as I used my N1's 3G quite a lot while on the road for my Ipod but I was never charged anything extra by T-Mobile for it.