The flight crew inadvertently entered an incorrect weight figure into the Flight Management System (FMS) computer. This resulted in the approach being flown at 136 knots instead of the correct airspeed of 151 knots. At this airspeed, the aircraft proceeded along the flight path at a pitch attitude of about eight (8) degrees instead of the three or four degrees that would have occurred at the correct airspeed. At this reduced airspeed and excessive attitude, the tail clearance during the flare was reduced to the point where the airframe contacted the runway surface.