I had a similar problem with my 12" PowerBook g4.
The cause: I kicked the computer's nightstand out from under it (cursewords!) and it fell about two feet landing it exactly on its right side.
The result: I got the "turn me off immediately!!!" screen. Subsequent reboot led to the fan coming on, the start up chime no no hard drive noises and a completely black screen.
The solution: I foolishly took the machine to the apple store's genius bar, where I was told it was at least a faulty hard drive and possibly display or logic board problems. Thinking this fix to be over my head, I paid 'em $300 (more cursewords!) to send the guy off to a mysterious place they called The Depot. Four days later, my PowerBook arrives in the mail, working wonderfully, with a note that says they simply RESEATED THE AIRPORT CARD! $300 for something I could have done with a quarter! They replaced no parts and reinstalled the OS. *sigh*
This fix makes no sense to me. Anybody know why an unseated or partly seated airport card would keep the display from coming on or the hard drive from firing up?