Q: Top Half of Screen Dark

Safe Boot, (holding Shift key down at bootup), is the top still dark?

Shine a flashlight at an angle & move it around at different angles to see if if you can see a dim Desktop on the dark part.

Is the brightness equal in Safe Mode?

Perform an Apple Hardware Test.  It might not find anything but it would still be woth running.

Intel-based Macs: Using Apple Hardware Test

I read on that link that for Lion the AHT is online, though how that works I have no idea. I am on 10.6.

Do you still have a clone of your 10.5 or 10.6 OS, because if you did you could boot into your clone and use the relevant DVD to test your hardware when booted into that same hardware environment.

I have this EXACT problem. The only way I've found to cope is to sleep/wake the display repeatedly until the top half stays at its original brightness. I would like to find a less insane solution.

It sounds like the backlight is failing.

Is there a way to fix this without disassembling the entire display?

Doubtful , it may require a new LCD panel. 

Pity. Thanks for the information!

It's pass a lot of time, but I found this post and I have the exactly problem. Did someone figured out with this?

Thanks