THE PILOT'S WX BRIEF INCLUDED A FLT PRECAUTION FOR OCCASIONAL MODERATE ICING CONDITIONS. LATER HE RECEIVED A PILOT REPORT OF MODERATE RIME ICING ALONG HIS ROUTE OF FLIGHT, AND AT HIS PROPOSED FLT ALT. THE AIRPLANE WAS EQUIPPED WITH A HEATED PITOT TUBE AND A HEATED PROPELLER. APPROX 1 HR AFTER DEPARTURE THE AIRPLANE STARTED AN UNCONTROLLED DESCENT FROM THE ASSIGNED ALTITUDE OF 9000 FT. THE PILOT REPORTED HE WAS UNABLE TO MAINTAIN HIS ALTITUDE DUE TO ICE AFTER THE AIRPLANE HAD DESCENDED OVER 1000 FT. HE WAS CLEARED FIRST TO 6000 FT, AND THEN 4000 FT, BOTH OF WHICH HE WAS UNABLE TO MAINTAIN. HE WAS LAST OBSERVED ON RADAR, DESCENDING THROUGH 2400 FT. A WITNESS ON THE GROUND SAW THE AIRPLANE IN A NOSE HIGH ATTITUDE WITH THE WINGS ROCKING FROM SIDE TO SIDE. HE SAID THE NOSE SUDDENLY PITCHED DOWN AND THE AIRPLANE DESCENDED INTO TREES FROM WHICH A FIREBALL WAS OBSERVED TO EMANATE. RADAR DATA SHOWED AN APPROX 15 KNOT REDUCTION IN GROUND SPEED, IN THE 5 MINUTES PRECEDING THE DESCENT.