At 0805 eastern time, the pilot of a Beech 1900D aborted takeoff of a positioning flight after maintenance due to a jammed elevator control. The two pilots were not injured and the airplane was not damaged. The maintenance performed on the aircraft required removal of the elevator trim wheel. The mechanic did not index the trim wheel before removal and reinstalled it so that when the wheel was positioned to neutral, the elevator trim was actually in a nose down position.