THE CESSNA WAS SUBSTANTIALLY DAMAGED WHEN IT NOSED DOWN AFTER ENCOUNTERING JET BLAST FROM A BOEING 737 THAT INITIATED TAXI FOR TAKEOFF. RAMP PERSONNEL MARSHALLING THE 737 FROM THE RAMP STATED THAT THE CESSNA WAS NOT OBSERVED UNTIL AFTER THE 737 MOVED AHEAD ABOUT 10 FEET AND STARTED TURNING TO THE RIGHT TOWARD THE TAXIWAY. THE RAMP PERSONNEL NOTED THAT THE CESSNA WAS WITHIN THE CONFINES OF THE AIRLINE'S MARKED PERIMETER AREA AT THE TIME OF THE ACCIDENT.