The solutions I've heard run from Black Magic to the useful.
The Black Magic Solutions:
- Clear PRAM (CMD/OPT/PR) (don't forget to fix the clock and network)
- Boot into Open Firmware (CMD/OPT/O/F) and type
- I can hold down the Option key to switch back to OS pre-X.
The most useful answer is: Run the System Disk application/control panel and reset the boot device. It's available at ftp://ftp.apple.com/developer/macosxserver/utilities/SystemDisk2.3.1.smi.bin
The real thing is that you need to change some setting in Open Firmware and/or the settings in the boot blocks on the drive. I've just verified that System Disk does the trick.