Then how is it that the top-left corner of my iPhone 4s says, “T-Mobile”?
I've had it for over a year. Bought an unlocked phone at an Apple store, stopped at the local T-Mobile store on my way home, oohed and aahed about it with the cute girl while she rummaged in the drawer for a micro-SIM, and made my first call on it in the parking lot before finishing the drive home.
It's with a fantastic pre-paid plan, too. I pay $2 / day on any day that I make even one phone call or transmit even one byte of network data, but everything is completely unlimited for the rest of that day. I telecommute, so I rarely use it for calls and even more rarely for data (thank to ubiquitous WiFi), which means I'm spending about as much over the course of a year as most people do in a single month. And I own the phone outright, with no lease or contract or any other restriction.
All on T-Mobile.