I hope you realize that the iPhone 3G will never be able to use T-Mobile's 3G network in the United States. AT&T's and T-Mobile's 3G networks use different radio frequencies, so the iPhone can't communicate with T-Mobile's 3G network. You might be able to get the iPhone 3G to work on T-Mobile's 2G/EDGE network, but I'm not sure if even that is possible.