You probably have a lid switch problem. Lid switch is responsible for switching off the filament of your LCD screen when you close the screen. Lid switch is also responsible for other functionality, those we won't discuss here as I want to focus on your related symptom. Filament is a thin CCFL tube inside your LCD screen which makes display visible to human eyes. When you close the LCD screen lid switch is pressed and your filament switches off. It exactly works like your refrigerator door switch . When you open the door you get the light and after closing the door light is turned off.
I think, your lid switch is stuck permanently or intermittently does not function as desired.
There are two types of lid switches used in notebooks ie mechanical and magnetic. You will have try and locate this switch and check for the switch functionality. To find out whether there is a magnetic switch installed in your system you need to locate a magnet in the LCD casing. That is very easy, take a metal piece and roll it around the border of your screen. And if your metal piece is attracted then bingo……………
Hope this was of some help.
Did you ever find the solution to this problem? Mine was working but just started doing this. I removed the magnetic switch and still failing. Actually, it was working for a while, but failing again. Feels software drivers messed up. Reinstalled many times, all kind of combinations... Going to flash BIOS to A6 version and next I am going to reinstall SP3. Running out of ideas...
I see this is a very old posting... But anything you can remember will help