During an instructional flight, while approaching the runway to land, the flight instructor told the pilot receiving instruction to check that the landing gear had been extended. While the instructor handled radio transmissions and watched an aircraft that was still on the runway, the pilot receiving instruction performed the prelanding checklist but did not extend the landing gear. Due to the distraction of the radios and traffic on the runway, the instructor did not notice that the landing gear remained stowed. During the flare to land, both pilots realized the landing gear remained up and the airplane settled onto the runway with the landing gear retracted. The airplane sustained substantial damage to the lower fuselage. Postaccident examination of the landing gear and landing gear position warning system revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation.