I had this problem when I first got my 3000. I've had it for about two weeks now and the light sensor works fine (as well as it could). For some reason, it did not active until I downloaded a new pcf file to my 3000. Ever since, it works.

Somebody posted earlier a great tip that it will not work if you place it directly underneath a lamp (mine is a halogen). I have tested and this is true. However, sitting in my room which has a 100w normal bulb on the ceiling definitely trigured the activation. Covering the light sensor with my hand turned on the backlight. Removing my hand turned the backlight off.