Let's first read a report from Aksam
Let's have a look at a details not mentioned yet. When looking at the specification for the US iPhone 3G (here), on can read "UMTS/HSDPA (850, 1900, 2100 MHz)".
If one look further at the specification of the 3G network in USA, one will immediately notice that only AT&T 3G network will be compatible with the iPhone 3G. According to technical specification of the US competitors, their 3G network will simply not work, making any unlocking of an AT&T iPhone 3G for being used with another carrier almost useless.
Indeed T-Mobile is the second largest US carrier with its own 3G network but based on a different bandwidth (1700/2100Mhz) instead of AT&T (850/1900Mhz), as indicated in T-Mobile wiki (http://fr.wikipedia.org/wiki/T-Mobile_USA). So indirectly
