I've tried 2 different ISOs (one that I burned and a copy from eBay) and tried nearly every bootloader option when I boot from the Live CD. All i get is colored horizontal lines and no text on the screen after I enter a boot option. I can hear the disc spinning like it's trying to load something but it seems there is something wrong with the graphics.
Try using the PowerPC version of Ubuntu. Intel versions won't work right with PowerPC computers.
These are the official links from cdimage.ubuntu.com .
Read the PowerPC support page on the Ubuntu Wiki for more information.