Earlier this year AT&T failed to acquire T-Mobile and as part of the breakup T-Mobile received wireless spectrum from AT&T. Until this point T-Mobile used the 1700/2100Mhz AWS frequency bands to provide 4G connectivity. Now, much to the joy of AT&T iPhone users T-Mobile is re-purposing 4G HSPA+ to 1900Mhz and ironically they're testing 1900Mhz in San Francisco's Moscone Center West.
This is where Apple holds a specific conference each year to announce a certain new device, hint iPhone 5. Although it's unlikely that T-Mobile will get the iPhone with a spokesperson saying it's "just coincidental", but at least anyone that has brought a iPhone to T-Mobile will enjoy 4G connectivity at the conference.