Many moons ago, I did exactly that with an AT&T phone. Got it, tried to make it work for awhile, but in the end returned it within 30 days (I didn't get service in my house or at my desk, where Verizon was fine). Fairly painless process.
I only remember one that had T-mobile that I knew of. He moved on due to issues with signal in Philly. He was also a big blackberry guy, not sure if that applies to you. I'm using Verizon, two lines, two phones, 4GB data, $153/mo..