Where does the T-Mobile iPhone get 3G/4G?

I've seen that the 1900 band receives HSPA+ in certain areas and that T-Mobile is planning on expanding HSPA+ coverage on the 1900 band.

http://goo.gl/9hx7A


Does anyone know where these areas are currently and when this expected to roll out nationally?

With the free AT&T unlock and my increasing love of iOS over Android, I'm very curious.


Thanks.