The pilot of the N427RD, a John Lingwall model Rebich-Pitts, experimental amateur-built airplane, stated that he was taxiing his single place aerobatic tailwheel airplane south on taxiway A for departure on runway 5. He stated that he made clearing turns, and mentally noted that there were airplanes in the run-up area. He said he observed one airplane taxiing into position for takeoff on runway 5, and believing that he had accounted for all the airplanes, he continued to taxi. He said that his airplane's propeller struck the right wing of another airplane, and it was at that moment that he realized there was another airplane that had been hidden from his view. The flight instructor and student on N970AC, a Piper PA28-16, stated that they were stationary in the run-up area, and provided additional information which concurred with the pilot of the Pitts. Damage to the Pitts was minor, and the Piper incurred damage to its right wing spar.