Colin Watson has kindly looked into the mirror archive error that has plagued the PowerPC live cds since 11.04. He thinks he has now fixed it, see https://bugs.launchpad.net/ubuntu/+s...or/+bug/919356 ,but he needs some help with testing.
The new choose-mirror package is in the alternate CDs and I think the changes should have made there way into the ubiquity (live CD) package by now. So if you live in an exotic (i.e. non UK) location then why not have a go at installing 12.04?!! Please give feedback in the above bug report.