At the Event Horizon chat on March 18, 1999, GRRM gave the following answer:

The Seven Kingdoms usage of course dates from the time of Aegon the Conqueror. At that time, there was the King in the North (1), the King of Mountain and Vale (2), the King of the Rock (3), the King of the Reach (4), the Storm King (5), the King of the Iron Islands, who also ruled the riverlands (6), and the kingdom of Dorne, which was ruled by a prince (7).