Ubuntu/PPC

Note: The following has been tested on a Mac iBook G4 (mid 2005)

Ubuntu 10.10 PowerPC

Use 'Startup Disk Creator' from the System > Administration menu from an i386/x86-64 Ubuntu installation (as the PowerPC version of Ubuntu doesn't support SYSLINUX, but the install CD contains the yaboot bootloader that is required, and it will be transferred to the USB).
Choose the downloaded Ubuntu Desktop 10.10 PPC .iso file or burned CD-ROM as the Source.

Read the precautions at this page, Boot PowerPC Macs via USB 2.0 drives
Power on while holding four keys down, option+Apple+o+f. This takes you into Openfirmware.
Enter devalias at the 0 > prompt (See step 4 in the above reference.)
Note the exact usb device addresses.
By experimentation, I found that my USB device would boot after setting the boot-device with this command:
setenv boot-device /pci@f2000000/usb@1b/disk@1:1,\install\yaboot
If the USB disc is not around, the default boot-device will boot your regular Mac installation.
Check your setting with this command:
printenv boot-device

Your system may have different addresses, so it may take a while to discover the proper address.

Use the Mac install DVD to resize and create partitions (including swap) for Ubuntu
Burn the Ubuntu installation CD and install Ubuntu into the new partition by booting the CD-ROM.
Use System > Administration > Additional Drivers to install the Broadcom B43 wireless driver.
Use Ubuntu Software Center to search for Sugar. Install Sugar, the emulator for the Sugar Desktop Environment, plus dependencies and Activities suitable for Sugar 0.88