At about 1040 eastern time, an amateur-built Lancair IVP was damaged during takeoff from Manassas Regional Airport. The pilot and passenger were not injured. The pilot said he received an IFR clearance with complex changes to his planned route. He then felt rushed and distracted during the pre-takeoff checklist and forgot to extend the flaps to the takeoff position. During the takeoff roll, the airplane became airborne, but then settled back on the runway, went off the right side, struck a runway sign and came to rest in a grass area.