Originally Posted by _Rick_V_
I bet the limitation is physical. Because T-Mobile operates at a higher frequency, the antenna design on the iPhone would have to be adjusted, which would compromise it's performance at lower frequencies. And put simply, Apple doesn't want to do that.
They could release a separate model. Plenty of other companies, with much less potential sells, still do it. So the economies of scale are definitely there.
The best case - Apple is working on 42Mbps HSPA+ model for T-mobile. I doubt it though.