Voicestream, which T-Mobile bought a few years ago, provided me with an unlocked phone right off the bat in 1999. Very nice of them, although they have good enough international service (I just txt my friends, rarely use it for voice calls overseas), that I've just stuck with my original SIM, now in its 4th phone.

The current phone that I have (ericsson t-68m), I bought from the amazon.com website, just tossed in the card & it was all set.