From the official Gran Turismo website:
This [Course Maker] feature, available at a later stage through an update, will allow you to create your own custom tracks that can be driven in the game. Additionally, in another update we will add the possibility to generate a track by capturing the GPS coordinate data of a mobile app while you are driving that course. This GPS-generated tracks will be available in the game as playable content.
With features like this coming to fruition, it's looking like GT6 is well on it's way to live up to the hype.
Hat tip to Greg Freestone!