I have the same problem with a G4 iBook.
I was working on a friend's iBook and everything was fine until today. My friend needed me to reformat/clean install the OS on it, which I did. It was up and running good for over a week, able to boot up and shut down normally several times.
Today, I got home from work and noticed that the screen was black, with the backlight on. Also, the caps lock and num lock leds were lit up, as well as the standby light. I attempted to power off the iBook, but it was unresponsive. I then proceeded to remove the power cord and battery. I then reinserted the power cord, but it would not power on. I put the battery back in, but it wouldn't power on from the battery either. If I leave the battery out for 5-10 seconds and then reinsert it, the caps lock, num lock, and standby lights would come on. I attempted a PMU reset using shift-control-option-power and I still can't power the unit on. I will be in deep if I can't get this unit back up and running. Please, please...I need help bad.