I'd assume that you'd have a rad over the bottom of the board and the motor housing that uses the air going by as it moves to dump the heat. At least that's my first idea, since I'm not a skateboard engineer it's probably not even close.
My experience with DC motors was that they got hottest when starting and stopping, that was the result of a high current demand for torque and relatively low operating speed with meant minimal airflow. If you're going up a hill you're easily burning 1 - 1.5 HP or a .75 - 1kW of power. They look very cool, can't wait to see one 'in the flesh' as it were.
