A BE90 at FL250 abruptly banked 30-40 degrees and descended 500 FT-600 FT as the pilot was turned to address passengers. The upset's cause is unknown.

Narrative

In cruise at FL250; autopilot on; I turned around to answer a question from the passengers in the back of the plane. As I was speaking with them the aircraft banked 30-40 degrees to the right and started an abrupt decent. I turned around to see what was happening and immediately pushed the Autopilot/YD Disconnect and corrected the aircraft altitude and heading and in that process made a proactive radio call to ATC to let them know something had happened and that I was correcting the issue. I informed them that no assistance was required at that time; and that I would call them back in a little bit to verify that all was OK. This gave me a chance to try and identify the problem. The aircraft had lost between 500-600 FT during this abrupt decent and had changed heading roughly 60 degrees. This aircraft is equipped with the G1000 package and a Garmin Autopilot. Because I hit the autopilot disconnect so quickly; in order to correct the aircraft's attitude; I am still unsure if it was an autopilot malfunction or a trim runaway. There were no conflicts with other aircraft.