If there is nothing wrong with the LCD screen when it does light up enough - such as - more than the few dead pixels that may have already been there before, or black lines, or lines of one color, or parts of the screen are not displaying, there's probably nothing wrong with the LC's and their circuits.
The screen being black does not mean there is anything wrong with the LCD assembly itself, outside of the inverter and CCFL.
The LC's - liquid crystals - do not produce light - they alter the color of or block the light coming through the screen from the CCFL. If the CCFL is burnt out you get no display at all, all the time.
You most likely candidate is the CCFL is starting to fail, and you just happen to be witnessing it - like street1 has pointed out, the symptoms are simlar to any flourescent light - when it's dying it behaves erratically.
I did some looking around a while back when a freind had similar problems, and I found some frequent symptoms and explanations for them on a web site that replaces CCFL's and invertors, or can sell you either.
In his case he replaced the inverter and it didn't help.
Notes I made, with stuff I later added:
1. The backlight is failing - if the voltage invertor for the LCD display backlight is okay, you may get a brighter display briefly while booting, then the display goes black or very dim and stays that way.
That happens because the voltage invertor circuit shuts off when it detects the backlight is poor to prevent frying itself. Another symptom of a failing backlight is the display probably had a pink or other colored tint before this blackness problem happened.
The backlight is fairly inexpensive (e.g. $40 on the web) but tricky to replace yourself (you must be very careful, and good at soldering - you have to remove the LCD assembly from the laptop lid and remove pieces from the LCD assembly edges) and it is recommended you have some technician who knows how to do that tackle it, which will cost you more, or you get yourself a complete LCD display assembly either refurbished (more expensive) or new (even more expensive).
The backlight is technically called a CCFL - a Cold Cathode Flourescent Lamp.
2. The other thing it could be is the voltage invertor is defective.
If it is dead, you will have blackness all the time.
The voltage invertor supplies high voltage to the CCFL.
It is relatively inexpensive (e.g. $45 on the web), and much easier to replace yourself than the CCFL is, or you can pay more to have someone replace it for you.
