If you get the correct data plan then they wont mind. Its a matter of finding the correct one. IF you want to learn a bit more about t-mobile's data plan setup then cruise on over to howardforums.com. Using unlocked non-t-mobile phones on their network has been going on for years. I myself do this all of the time. Due to the advanced nature of the non-t-mobile phone I use alot more data than I ever could with one of their pieces of garbage. I've been doing all sorts of net surfing and yes, even youtube (despite what apple says, they were not the first to get this).
One new company, MyGallons.com, lets customers buy gallons ahead of time based on the current prices in their area--in theory, locking in a lower price--and then redeem those gallons at a later date, regardless of any price increases in the interim.