okay.. so, here's an update from a T-Mo customer rep...
i was supposed to call them today regarding my screwed up account, and also wanted to let it be known that i have no coverage at home. when i mentioned the nearest spot i get data service is at the MetroPCS store down the street and that it was because they probably have a booster there, they responded that they could send me a booster free of charge (they really want to keep me as a customer). i let them know that i also travel around the country for work and that i need to see how it is on the road. i didn't want to accept the booster (since i was already planning to leave them and get my phone unlocked to use on Sprint since they're the only provider that offered such a thing while continuing the financing with them) only to complicate my already screwed up account with them and continue further charging of my cc.
so, i told her i'd give it a few just to see how it goes, just something to document into the notes. she said i could cancel at anytime within the first 30 days and return the handset for my money back (including money spent on service, just everything) or, continue financing or pay it off completely and get it unlocked to use on (her examples...) Sprint or Verizon. so, i asked if that meant for international carriers or domestic. she responds that there are two types (what Coleman mentioned above): temporary for international use and permanent. she repeats saying that should i decide to cancel within the 30 days, i can cancel my service with them and they will permanently unlock it to use on Sprint - it's the same level of unlocking between buying their phone at full price or only continuing the payment plan with them.
i asked her that if i do end up cancelling the service with them within the first 30 days but continue the financing with them, if i can't get my handset to work on Sprint, if i could still return it for my money back. she said yes, if within the first 30 days. so, i'll be trying that. if i have any issues, i'll just return the dang thing.
then when i was transferred to T-Mo tech support specialist to go through my phone settings to make sure it's optimised, then went into the same details about being able to get it unlocked to use on Sprint. we'll see...