THE INSTRUMENT RATED PLT INITIATED VFR FLIGHT TO BLACKSBURG AFTER HE WAS INFORMED BY FSS THAT IMC EXISTED AT THE DEST. THE PLT CONTACTED ROANOKE (ROA) APCH CTRL ABOUT 15 MILES WEST OF BLACKSBURG AND 8 MILES SW OF ROA. HE REPORTED THAT HE WAS 'STUCK' IN CLOUDS AT 2,000 FT MSL AT THE SAME TIME WITNESSES IN THE AREA REPORTED AN ACFT JUST ABOVE TREETOP LEVEL IN HEAVY FOG AND BELOW AN ESTIMATED 100 TO 200 FT OVERCAST. THE PLT WAS CLEARED TO AND BEGAN CLIMBING WEST TO 5,000 FT. AS HE WAS CLIMBING HE ASKED FOR THE CURRENT WEATHER AT BLACKSBURG. ROA ADVISED THE PLT THAT THE WEATHER AT BLACKSBURG WAS NOT AVAILABLE. HOWEVER, THE PLT ASKED AND WAS CLEARED TO PROCEED TO BLACKSBURG ON A 240 DEGREE HEADING. THE ACFT WAS AT 4,600 FT WHEN IT ABRUPTLY MADE A RIGHT TURN AND DESCENDED TO THE GROUND. WITNESSES STATED THE ACFT DESCENDED FROM A LOW OVERCAST IN A NOSE DOWN ATTITUDE WITH THE ENGINE AT HIGH POWER. EXAMINATION OF THE ACFT DID NOT REVEAL EVIDENCE OF MECHANICAL MALFUNCTION. THERE WAS NO EVIDENCE OF PLT IMPAIRMENT OR INCAPACITATION.