Here is the leaked list of when certain cities will be getting the 3G network from T-Mobile.

The full leaked list (including cities recently launched):

Baltimore – Aug 18th
Houston – Aug 19th
Minneapolis – Aug 20th
San Diego – Sept 15th
Los Angeles – Sept 16th
Phoenix – Sept 16th
Sacramento – Sept 17th
Portland – Sept 18th
Seattle – Sept 23rd
San Francisco – Sept 24th

[Via CellPhoneSignal]