Sprint is launching LTE in Hampton Roads tomorrow!! Just noticed my S3 was connected to 4G at my house today, too. I believe there are a couple towers that are turned on in williamsburg that are not on the update map. Specifically 4881 centerville rd; and powhatan creek trail, williamsburg.
http://www.engadget.com/2013/04/11/sprint-leak-has-lte-reaching-los-angeles-april-12/