Your system's first boot on its own power is what electrical engineers call the “smoke test”.
If you are booting directly into Ubuntu, and the system doesn't start
up, either use your original installation boot media, or insert the
custom boot floppy if you have one, and reset your system. This way,
you will probably need to add some boot arguments like
root=, where
rootroot is your root partition, such as
/dev/sda1.
On G4 machines and iBooks, you can hold down the option key and get a graphical screen with a button for each bootable OS, Ubuntu will be a button with a small penguin icon.
If you kept MacOS and at some point it changes the OpenFirmware
boot-device variable you should reset OpenFirmware to
its default configuration. To do this hold down the command-option-p-r keys while cold booting the machine.
The labels defined in yaboot.conf will be
displayed if you press the Tab key at the
boot: prompt.
Resetting OpenFirmware on G3 or G4 hardware will cause it to boot
Ubuntu by default (if you correctly partitioned and placed the
Apple_Bootstrap partition first). If you have Ubuntu on a SCSI disk
and MacOS on an IDE disk this may not work and you will have to enter
OpenFirmware and set the boot-device variable,
ybin normally does this automatically.
After you boot Ubuntu for the first time you can add any additional
options you desire (such as dual boot options) to
/etc/yaboot.conf and run ybin
to update your boot partition with the changed configuration. Please
read the yaboot HOWTO
for more information.