After accelerating through the V1 speed during takeoff the captain raised the nosewheel from the runway. Suddenly the nose undercarriage retracted and the aircraft sank back, contacting the runway with its nose. The captain retarded the throttles and the plane slid to a stop 272 feet beyond the runway end. PROBABLE CAUSE: “Premature retraction of the nosewheel assembly during the takeoff roll for an undetermined reason.”