I think the partition to boot is getting lost in the open firmware. I do not know how or why. You can check on this.
On of the variables contains what to boot.
List of variables:
printenv
Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime.
Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
reset-nvram
set-defaults
reset-all
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."
http://support.apple.com/kb/TS1812?viewlocale=en_US
Should the fail...
Try taking the battery out for 10 minutes. Put battery back in. Cross fingers. Power the machine back on.
How to eject a cd from the internal cd drive:
eject cd
List of devices:
devalias
List of variables:
printenv
More than you ever wanted to know about open firmware
http://www.firmworks.com/QuickRef.html