Unofficial ports are made exclusively by volunteers, and always lag a bit behind. So I guess you just have to be patient.
official version of the PowerPC desktop CD Natty had a tremendous Ubiquity bug in the installer had no possible to create a repository configuration to update and complete the installation and pre-installed base in the previous steps of the installer, so in many cases not even ended up creating a user account with which to start a session, leaving practically for expert users to correct these bugs in ubuntu with console to access the desktop of Ubuntu 11.04, but if you want this CD version of "natty-desktop- powerpc.iso "you search the web if any mirror somewhere in the world still has it for reasons of history for cases like yours, in this case I found the version natty in Kubuntu (KDE desktop) in the mirror of the university Harvard (23-June-2011): http://mirrors.med.harvard.edu/ubuntu-cdimage/kubuntu/daily-live/current/, I hope that this version does not have the same problem as the natty Ubuntu version ( gnome desktop), and I think that the installer problem was the reason to withdraw the PowerPC line Ubuntu 11.04
I found a CD ISO in this link for Oneiric 11.10 on powerpc, I am downloading it now, so...
Well, this is the link: http://mirrors.med.harvard.edu/ubuntu-cdimage/daily/20110926/