TWO AND A HALF HOURS BEFORE THE FLT, THE PLT OBTAINED A WEATHER BRIEFING FOR A VFR FLT. THE SPECIALIST REPORTED FLT PRECAUTIONS FOR MOUNTAIN OBSCURATION WITH OCCASIONAL MODERATE RIME OR MIXED ICE IN THE CLOUDS. JUST PRIOR TO THE FLT, THE PLT FILED A VFR FLT PLAN AND CHECKED THE WEATHER AGAIN FOR CHANGES. WHEN THE PLT CONTACTED MISSOULA RADIO TO OPEN THE FLT PLAN, THE PLT CHANGED THE VFR FLT PLAN TO AN IFR FLT PLAN. AFTER TAKEOFF, THE PLT WAS CLEARED TO 11,000 FT. SHORTLY AFTER THE FLT WAS LEVEL AT 11,000 FT, THE PLT REPORTED THAT HE WAS PICKING UP A LOAD OF ICE AND REQUESTED A LOWER ALT. THE PLT WAS CLEARED TO DESCEND AND MAINTAIN 9,500 FT. A SHORT TIME LATER, THE PLT NOTIFIED THE CONTROLLER THAT HE WAS PASSING THROUGH 8,500 FT AND UNABLE TO MAINTAIN ALT. WITH THE USE OF ANOTHER AIRPLANE IN THE AREA TO RELAY MESSAGES, THE PLT REPORTED THAT HE WAS IN IMC. NO FURTHER COMMUNICATIONS WERE AVAILABLE. AN INITIAL SEARCH WAS UNSUCCESSFUL BUT THE WRECKAGE WAS FOUND NEARLY 4 MONTHS LATER IN MOUNTAINOUS TERRAIN.