CrackBerry Genius of Geniuses
To quote Rene
"While also a GSM/HSPA/HSPA+ network, T-Mobile iPhone 5 would have to work on AWS (Advanced Wireless Services, which operate in the 1700 and 2100 bands), a very different set of 3G frequencies than AT&T. Whether or not Apple could combine all those frequencies into one GSM iPhone 5 antenna array, never mind a single CDMA/GSM “world phone” that could work on Verizon, Sprint, AT&T, and T-Mobile, is the question."
This is where she got the information from.