THE ACFT WAS OBSERVED TO BE FLYING SLOWLY, THEN ROLL STEEPLY TO THE LEFT AND DIVE INTO AN UNOCCUPIED HOUSE FROM ABOUT 300 FT SHORTLY AFTER TAKEOFF. THE PLT HAD CONSIDERABLE FLT TIME BUT HE HAD NOT FLOWN THIS MODEL, EXCEPT FOR A 35 MIN CHECKOUT THAT MORNING. THE CHECKOUT CONSISTED OF 3 TAKEOFFS & LANDINGS. THE PLT'S FRIEND, A RATED PLT WITHOUT A MULTIENGINE RATING, SAT IN THE RIGHT COCKPIT SEAT & HAD LOGGED FLT TIME IN THE PLT'S OTHER TWIN AIRCRAFT. A WITNESS STATED THE PLT LOOKED PALE & APPEARED TROUBLED JUST BEFORE THE FLT. THE AUTOPSY OF THE PLT REVEALED THE LEFT MAIN CORONARY ARTERY WAS BLOCKED EXCEPT FOR A 'PINHOLE' OPENING. THE DOCTOR STATED THE CONDITION COULD HAVE CAUSED A SEIZURE BUT COULD NOT SAY WHETHER IT HAD ANY BEARING ON THIS ACCIDENT. NO EVIDENCE COULD BE FOUND TO INDICATE ANY PREIMPACT MALFUNCTION OR FAILURE WITH THE AIRFRAME, PROPELLERS, OR ENGINES. THE ACFT WAS ESTIMATED TO BE 134 POUNDS OVER GROSS WEIGHT.