This is true. The 3G (UMTS) networks of AT&T and T-Mobile operate on different frequencies. AT&T uses predominantly 1900 MHz (and 850 MHz), while T-Mobile US uses 1700 and 2100 MHz. In Europe 3G is at 2100 MHz. The 3G T-Mobile US network is one of very few 1700 MHz 3G networks in the world.

Presumably the current iPhone 3G is triband (850/1900/2100) as it is sold in Europe, Australia, South America, and North America. A quadband (850/1700/1900/2100) iPhone would work on T-Mobile in 3G, but there is little reason before 2010 for Apple to include 1700 MHz capability.

http://en.wikipedia.org/wiki/List_of_UMTS_networks#North_Ame...