The Maryland Grape Growers Association recently updated its survey of Grape Growing in the state. While obviously not all inclusive since it only received inputs from MGGA members, it is the most complete picture available of what is being grown in the state and by commercial or non-commercial interest.
You can get the survey results here
http://marylandwine.com/mgga/vineyards/stats.html
Special note to Paul B. Notice that Chambourcin is expected to rise to the 5th most widely planted variety in the state increasing in plantings by 75% this spring.