A Boeing 747-406 passenger plane sustained substantial damage to the horizontal stabilizer when it struck a de-icing truck. The airplane was being de-iced on the J-platform at Amsterdam-Schiphol International Airport (AMS). The de-icing supervisor had reported to the crew that they were finished and cleared the flight to taxy out for departure. The supervisor failed to notice that one truck was still active near the horizontal stabilizer. The stabilizer contacted the truck, causing it to fall over, injuring two workers.