What is the technical 'cause' of the infamous iMac black screen problem?

Can anybody shed some light on what technical malfunction exactly causes the infamous iMac black screen problem? The common solution seems to be a logic board replacement. But what exactly is broken on the logic board? Is it the Geforce chip? Some kind of external driver circuitry? Or something completely different?

I was referring to the many problems also described in this thread (and also in many other places):
http://discussions.apple.com/thread.jspa?threadID=196774&tstart=0
In specific I am referring to machines where all of this applies:
- internal monitor stays black after power-on
- computer boots normally
- external display works normally
- internal display backlight works and can be adjusted in brightness, but all pixels are black
- PRAM reset doesn't help
- NVRAM reset doesn't help
- PMU reset doesn't help
- reinstalling OS / booting from CD doesn't help
- machines with this problem sent to repair come back with a replaced logic board
I'm looking for more information on the technical cause of the exact problem as stated above. So I can attempt to repair my iMac

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