I tether on Sprint with a palm 700p. Pretty reliable connection. I connect with it as soon as in car and leave on. Not sure if I am ALLOWED to but I do under the $15 data pack plan(In addition to the Minutes plan I share with my wife). I didn't even need the hacks that are out there, I just paired them up and the Palm even has a little computer icon in the bluetooth status area when tethered.

So far, the time I am tethered is not showing up on bill in minutes used or kb downloaded, so not sure what is going on there