I'm a Dutch iPhone user, which means we are locked into T-Mobile contracts like you American's are with ATnT and when I was in the US with my iPhone 3G, surprise surprise I was on the T-Mobile 3G network. Our Dutch phones are sold and software configured for the multiple frequencies used by T-Mobile on the Dutch and German GMS networks. In Holland I know they use 1800 for sure, as well as other lower frequencies, so when my iPhone was in the US, 1700 T-Mobile 3G, own carriers network? No problem, although it did switch to ATnT when they had better coverage 😛.
The only reason American iPhones may not go one the network is simply because you receive different carrier settings which is determined by your geographical location + iTunes account. For example, I noticed with my uncles Belgium iPhone went Swiss when he restored it on holiday in Switzerland with a freshly made iTunes account even though the Sim card and computer were Dutch, the phone was Swiss. The iPhone could probably technically be rolled out tomorrow on the US T-Mobile network, though obviously there are other factors involved in such a decisions for Apple.