If its a phone, you'd do better to use Wifi. Get a phone with Wifi, you'll be grateful.
If its a laptop with a cellular module, you shouldnt even bother. Just use Wifi. Its free. Of course, if you get the MyTouch you'll be forced into a special data plan with unlimited usage, so cost wont really be an issue. But speeds are apalling. Use the Wifi. T-Mobile is one of the few companies thats cool with unlocked phones, but on the G1 and My Touch, some things HAVE to be done over cellular data, so if you got an unlocked android phone its unlikely you'd be able to do EVERYTHING over Wifi and not have a regular data plan, but you would wanna use it whenever possible.