I'm sure your local mac expert tried this... but just in case, try zapping PRAM. I have had a Mac which wouldn't boot of CD, disk, whatever... turned out the PRAM was corrupt... If I recall correctly, that was just after hot-swapping a CD-R.
And there is also the small push-button on the PCB that is even better to zap PRAM than the key combo. It is somehwere in the back middle 1/9 of the PCB, and is red or black.
.x. ... ... Front - X marks approx area where to look for it if I remember right.