T-Mobile may be transitioning 1900 band to HSPA+16
There have been some reports that users with unlocked iPhones running on T-Mobile have been picking up 3G coverage in some areas. The thing is that T-Mobile's traditional 3G band (1700 Mhz AWS) doesn't exist in the iPhone radio. The theory is that T-Mobile has been transitioning its 1900 Mhz band, which it uses for 2G coverage, to build up the HSPA+ network.
This could be part of T-Mobile's strategy to push its HSPA+ network past the current 42 Mbps speeds into the 84 Mbps territory. LTE or not, that's some fast (theoretical) speeds.
source: Howard Forums via TMo News