The Installer CD you are trying to boot from, is a daily version, so it can still be not 'ready' - it was still buggy the last time a tried it. You may want to try the Intrepid alternate installer cd (8.10) http://cdimage.ubuntu.com/ports/releases/intrepid/release/
or you can download the live CD of Hardy (8.0.4) http://cdimage.ubuntu.com/ports/releases/hardy/release/
If you decide to use the alternate installer of Intrepid, be aware that you will most probably have a small problem, that you will solve like here http://ubuntuforums.org/showthread.php?t=964904
………………… Here is a list of all releases https://wiki.ubuntu.com/PowerPCDownloads
………………… If you don't have much experience with linux/ubuntu, I would definitly go with Hardy (8.04). Can you please tell us more about your Hardware (memory, HD, any PCI card…)
Before installing, you can read this helpful links https://wiki.ubuntu.com/PowerPCFAQ https://wiki.ubuntu.com/PowerPCKnownIssues