THE PLT HAD REQUESTED A VFR DEPARTURE EASTBOUND TO WATERVILLE & RADAR FLT FOLLOWING. HE STATED HE WOULD BE GOING VFR THROUGH STAMPEDE PASS. AFTER TAKEOFF THE ACFT WAS TRACKED ON A RADAR HEADING OF ENE AT AN ALTITUDE OF 2,500 FT MSL. SHORTLY THEREAFTER THE PLT CANCELLED FLIGHT FOLLOWING, STATING HE CAN MAINTAIN VFR & WOULD BE STAYING LOW. THE ACFT IMPACTED RAPIDLY UPSLOPING TERRAIN 1 MI NORTH OF INTERSTATE 90 AT 3,950 FT MSL. THE SITE IS LOCATED ON THE NORTH SIDE OF A VALLEY THROUGH WHICH I-90 PROCEEDS GENERALLY EASTBOUND. THE APPROXIMATE MINIMUM LATERAL DISTANCE ACROSS THE VALLEY AT THE 4,000 FT LEVEL IS 12,000 FT. THE 1555 PDT WX RECORDED BY THE STAMPEDE PASS WX OBSERVATION FACILITY, LOCATED 15 MIFROM THE ACCIDENT SITE & AT AN ELEVATION OF 3,800 FT, WAS CEILING INDEFINITE OBSCURATION (AT THE SURFACE) & 1/8 MI VISIBILITY IN FOG. THE FLAP HANDLE WAS FOUND TO BE PULLED UP INTO A POSITION OF PARTIAL FLAP EXTENSION.