[citation][nom]house70[/nom]Is a proto, means not yet in production. no jailbreak needed, the phone is factory-unlocked. and no need for third-party apps. They're just testing the radio.[/citation]
A change of SIM does not work because T-Mobile runs on AWS1700, and AT&T doesn't. It's the same reason Canadian Wind and Mobilicity customers cannot currently use the iPhone on their respective networks whereas Rogers, Bell, and Telus customers can since they use the standard UMTS/HSPA network that most of the rest of the world, including AT&T, uses. I may be mistaken, but I believe that if Apple wanted to, they could use the current 1X/EVDO iPhone 4 used by Verizon for Sprint as well without modification.