On September 27, 2001, at 1330 mountain daylight time, a Cessna 172RG, N9320D, was substantially damaged during a gear up landing at Centennial Airport, Englewood, Colorado. The flight instructor and his commercial certificated dual student were not injured. The aircraft was being operated by Key Lime Flights, Englewood, Colorado, under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local instructional flight which originated approximately 40 minutes before the accident. The flight instructor said that he remembered seeing the red transition light of the landing gear position indicator turn off, but does not remember seeing a green down-and-lock light. Both pilots reported that they did not hear a gear warning horn when the throttle was pulled full aft for the landing. The airplane landed with the landing gear in the "up" position. The horizontal stabilizer was bent up when it struck a sign. Postaccident operation of the landing gear system indicated that the system was functioning normally.