This aircraft, which was departing on a scheduled flight to Lewiston, ID., was just accelerated to rotation speed when the crew heard one of the passengers scream. The takeoff was successfully aborted and the aircraft was returned to the gate. Investigation revealed that the right overwing emergency exit door was missing. The door was located about 3000' from the departure end of runway 10L. According to the passenger seated adjacent to the exit, she noted when she boarded, that the plastic safety cover was broken and the T handle activation mechanism was slightly ajar. She also felt a draft around the door. The crew did not notice any irregularities with the exterior handle when they did the preflight. Examination of the door revealed that the locking pin rigging was correct and the pin receptacle appeared normal. Tests show that once the pins are retracted from their normal overcenter positions, the door will fall free of the frame outboard. This aircraft is not pressurized.