Why not simply use what is used on real alarm systems; that is, a passive IR switch sensor?
3.3V TTL output (so some level conversion would be needed); place it in the corner of the room diagonally opposite that of the door, about 10-15 cm below the level of the ceiling and angled down toward the door. It'll cover the majority of the room, and can be made fairly sensitive. Bonus: You can use it when not in alarm mode to automatically turn on and keep lights on while you are in the room (well, until you stop moving - you need to have a delay to check for motion in the room to reset the time-to-live on the lights going out).