The upgrade has been announced for Phoenix, Mesa, and Tucson, AZ.; Modesto, Oakland, San Francisco, San Jose, and Stockton, CA; and Miami, and Fort Lauderdale, FL. The strange thing is that nowhere in the announcement is the 1900Mhz band mentioned. Instead, the announcement says that those living and travelling to these areas "will benefit from enhanced voice and data coverage, and faster speeds on unlocked devices".
That last phrase is key, because the 1900Mhz band is what allows for HSPA+ speeds on any unlocked device from AT&T, and most international unlocked devices, like the Nokia Lumia, and most notably the iPhone.
This announcement brings the total number of 1900Mhz band enabled markets to just 15 for T-Mobile, but there have been unofficial reports of it being turned on in other areas, as noted by this map.