Okay, found a bit more time to have a look into this. The easiest thing is probably to install a newer version of unity. There should be a newer version in oneiric-updates, but maybe it has been missed off for powerpc (EDIT: yep here is the fail https://launchpad.net/ubuntu/+source...+build/2875419, but it was built later for 12.04)? The powerpc time stamp is a few days later than for other architectures. This is what I think is causing the problems. So see if you can install the new version manually....
Then you will be able to install ubuntu-desktop.
wget -P/tmp http://ports.ubuntu.com/pool/main/u/unity/unity_4.24.0-0ubuntu2b1_powerpc.deb
sudo dpkg -i /tmp/unity_4.24.0-0ubuntu2b1_powerpc.deb
EDIT: This fix nolonger works because the unity package in 12.04 has been upgraded. You can download it directly from launchpad instead https://launchpad.net/ubuntu/+source...+build/2882663 , or install an older version of unity-common:
You will then be able to install ubuntu-desktop. There is also a new unity package in the proposed repository that you could alternatively enable or download as above? See https://launchpad.net/ubuntu/+source/unity and http://ports.ubuntu.com/pool/main/u/unity/ .
wget -P/tmp http://ports.ubuntu.com/pool/main/u/unity/unity-common_4.22.0-0ubuntu3_all.deb
sudo dpkg -i /tmp/unity-common_4.22.0-0ubuntu3_all.deb