I did a bad thing and got an Iphone and left T-Moble and the smartphone I got free a year prior, since T-Mobile didn't get service in my new home. I called them to cancel, expecting to get a 200$ termination fee, but when I told them why I was canceling, they waived the fee due to my address not being in their coverage area. all they asked was I send them a copy of a bill that shows my name at that address.
Thanks to this treatment, I'll be moving back to T-mobile when/if they ever get 3g coverage in my area. It seems to me that burning your customers on the way out is a sure way to ensure they NEVER come back.