I've been following this from afar without the ability to reply, but it's been interesting reading!
Now that I'm back, the only suggestion I can add to the mix from the top of my head would be mains powered lamp pointing at a webcam, which given the contrast difference between light on and light off ought to be fairly possible to determine programatically whether it was on or off without being too clever. If there's an existing webcam (which I believe there is but I'm not sure it's available to the eeePC?) then this could probably be used but might be more fun to get working (eg if there's a light on in the room that the camera is monitoring and you have access to the images, then the contrast difference between light on/off should be detectable).