I am Canadian and on the carrier Telus, I use an iPhone 3GS. When I crossed the border into the United States today, I noticed that my carrier became T-Mobile, which I thought was kind of neat considering they don't do iPhone? Then I hit a dead zone for half an hour and was put on USA Unicell for a bit, and then I had a dead zone for an hour and a half and was put on AT&T and have had five bars on that ever since. But it appears that through Settings > Carriers, I can choose between T-Mobile and AT&T at my whim? Not really sure how this works.