A BE40 EXPERIENCED AN UNCOMMANDED ROLL TO 90 DEGS WHILE DSNDING THRU 15000 FT AT 290 KTS WITH THE AUTOPLT ON.

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WE WERE FLYING AT FL400 RTE ALT; AND WE WERE DSNDING THROUGH 15000 FT WITH THE PWR IDLE AT 290 KIAS; AUTOPLT IN COMMAND 'ON' VNAV MODE ON THE ARR TO 12000 FT AND 250 KTS. WE FELT A SLIGHT BUMP AND SUDDENLY AND VIOLENTLY THE ACFT ROLLED 70 DEGS TO THE R. THE ACFT THEN CONTINUED TO ROLL TO ALMOST 90 DEG POS. I IMMEDIATELY DISENGAGE THE AUTOPLT TO REGAIN CTL. IT TOOK A HUGE AMOUNT OF OPPOSITE CTL INPUT TO RETURN TO A WINGS LEVEL POS WITH ASSISTANCE OF MY COPLT. WITH THE ACFT BACK UNDER CTL 'WINGS LEVEL POS' WE CHECKED ALL THE GAUGES AND INDICATIONS TO MAKE SURE THAT EVERYTHING WAS OPERATING PROPERLY AT THAT TIME OUR ALT WAS 12200 FT AND 240 KTS. I NOTICE A VIBRATION IN THE FLT CTLS AND A NOISE 'UNFAMILIAR TO US' COMING FROM THE R SIDE OF THE ACFT. I THEN ENGAGE THE AUTOPLT TO VERIFY THAT IT WAS OPERATING PROPERLY. AT THAT TIME IT WAS. I DISENGAGED THE AUTOPLT AND FLEW MANUALLY FOR THE REST OF THE FLT. AT THAT TIME THE FLT CTLS FELT VERY SLUGGISH TO ME -- 'MORE THAN USUAL.' WE CONTINUED TO ZZZ 'THE NEAREST SUITABLE ARPT' AND LANDED WITHOUT INCIDENT. THE WX CONDITIONS FOR THE ENTIRE FLT WERE VFR; AND AT NO TIME DID WE ENCOUNTER TURB OR ICING CONDITIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: AT THE PRESENT TIME THE UPSET'S CAUSE IS UNDER INVESTIGATION. THE ACFT HAD NO DFDR INSTALLED IN ORDER TO HELP UNDERSTAND THE FLT CONTROL INPUTS BUT SOME STRESS DAMAGE WAS DISCOVERED IN THE CONTROL SYSTEM LINKAGES TO THE HORIZONTAL STABILIZER. THE REPORTER IS NOT AWARE OF ANY ACFT IN THE IMMEDIATE VICINITY AT THE TIME OF THIS EVENT THAT COULD HAVE GENERATED WAKE TURBULENCE RESPONSIBLE FOR THE UPSET.