What could be wrong with iMac display backlight other than CCFLs?

My iMac 20" (early 2008) recently lost all backlight. Here are the symptoms:

1) On startup, and on wake, the screen reaches full brightness for less than a second, then goes dead. This is why I think the CCFL's themselves are not the problem, but rather something that is supposed to be powering them.

2) For the past year, the screen simply went dimmer than usual, rather than going out completely. So I suspect that something half-broke a year ago, and is now fully broken.

3) Shining a light on the screen reveals that the LCD itself is working just fine. Also, my external monitor works, so at least the video card is probably okay.

Things I've already tried:

Resetting the SMC, PRAM and NVRAM

Are these symptoms consistent with a specific known problem? Is there anything else I could do to try to diagnose what the issue is? I'd rather not start replacing parts that aren't necessarily broken.