I may have (had) this same problem ... until I got it fixed today. My "fix" may provide you with some insights if your case is similar to mine, i.e. screen suddenly goes dark while I'm using it (like 99% dark - you may still see some faint outline of what was on the screen before it got dark) ... and tilting the screen may (or may not) bring the screen back (to normal) ... and you know the program (or music) is still running (while the screen is black). For the past few months, I would tilt the screen at various angles hoping to get the screen (backlight) on again.
I called Apple Support, and they wanted $49 before they start to diagnose the problem (over the phone) because my MacBook is out-of-warranty. (I bought it in June 2008). Anyways, I searched the web and figured out that the inverter board and/or cable (to the screen - on the left hinge of the screen panel) is the problem. It seems like a common problem with MacBooks. So, I brought it into an Apple store (Genius Bar) and got these parts replaced. It's a simple job ... and it did the trick.
If you're a do-it-yourself person, you can get the parts (from Apple) and replace the components yourself. For some ideas of how to disassemble your MacBook (or other Apple computer), check out:
http://www.powerbookmedic.com/mac-repair.php
If you only need to replace the inverter board, the part costs $49. The cable costs $9. Labor may be extra, but it's something that can be done within an hour.
