THE PLT ENCOUNTERED AIRFRAME ICING WHILE EN ROUTE DURING A X-COUNTRY FLT. HE HAD FILED AN INSTRUMENT FLT PLAN TO BOISE, ID, BUT EN ROUTE, HE CHANGED DESTINATIONS TWICE, 1ST TO ONTARIO, OR, THEN LATER TO BAKER, ID. AT 1820 PDT, HE REPORTED ICING & TURBULENCE AT 10,000 FT. HE WAS CLEARED FOR AN APCH TO BAKER, DURING WHICH, HE REPORTED MORE ICING. AT 1850, HE REPORTED GROUND LIGHTS IN SIGHT. A SHORT TIME LATER, THE BAKER FSS SPECIALIST SIGHTED A FIREBALL, NORTH OF THE ARPT. THE ACFT WAS FOUND 7 MILES NORTH OF THE ARPT NEAR A HIGHWAY REST STOP WITH HIGH VISIBILITY, SODIUM VAPOR LIGHTS. AN EXAM OF THE WRECKAGE REVEALED THE GEAR HAD BEEN EXTENDED, BUT THE FLAPS WERE UP. THE PROP WAS EMBEDDED IN A CRATER. THE PROP SHAFT HAD SHEARED, BOTH BLADES WERE SLIGHTLY BENT & HAD CHORDWISE SCORING & LEADING EDGE NICKS. THE MAIN WRECKAGE CAME TO REST NEXT TO THE IMPACT POINT. BOTH WINGS HAD SEVERE CHORDWISE CRUSHING FROM FORE TO AFT.