After a 30-minute flight, the pilot landed his single-engine tricycle gear homebuilt airplane at a private 3,500-foot long, by 50-foot wide grass airstrip to attend a fly-in. The event hosted numerous airplanes, many with a similar tricycle gear design. The 4,300-hour airline transport pilot reported that the airplane touched down on the main landing gear at approximately 60 mph with full flaps. The pilot added that shortly after touch down, during the landing roll on runway 02, the nose landing gear contacted "an irregularity in the landing surface" which caused the nose landing gear assembly to fold aft. Following the collapsed of the nose gear assembly, the propeller struck the ground and the airplane nosed-over coming to rest in the invested position. The canopy was crushed; however, both occupants were able to egress the airplane unassisted. Several of the pilots attending the fly-in witnessed the accident and later assisted in the recovery of the airplane from the runway. A couple of the pilots, who were also builders of the same model of airplane, were able to evaluate the condition of the landing surface, and failed to find any irregularities with the landing surface that could have contributed to this event. Another witness stated that the grass runway featured a slight 'dip' or grade and the touchdown zone of the runway "may have been slightly rougher that the rest of the runway." The homebuilt airplane had accumulated a total of 264.1 hours since new, and the pilot reported having accumulated a total of 264.1 hours in the make and model airplane. In the recommendation section of the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot recommends an "improved design for the tricycle-gear Vans aircraft." The homebuilt airplane is available in two options, tricycle or tailwheel configuration. A review of the accident data base revealed several similar events involving the tricycle gear configured airplanes. The reason for the failure of the nose landing gear assembly could not be determined.