I have a PowerBook G4 that was previously running Debian squeeze, and before that, OS X Leopard. It worked fine under Debian, but I disliked the GUI, so I decided to give Ubuntu a try.
I downloaded a copy of the PowerPC Desktop ISO, but the GUI was messed up: most of the screen was taken up by a large white rectangle, which was presumably supposed to be a window, but didn't load properly. Nothing happened when I clicked on the menus at the top, except for the one in the top-right corner that has shutdown options. I tried to restart it using that, but I didn't get a dialog or anything, so I finally ended up using control-command-power to force a reboot.
I then tried the alternate ISO to see if I could just use the textual installer. That worked, and I don't remember having any problems with the login screen, but after that, I had the same problems as with the live disc. I had no problems using the tty's, though, which is mostly what I did under Debian.
I later found that if you want full-disk encryption, you must set it up at install time. So I rebooted to the alternate disc and wiped the entire hard disk again, choosing the
entire hard disk with encrypted LVM option during partitioning. The installation completed successfully, but now when I try to boot, I get just a black screen after the
yaboot menus, no password prompt or anything. How can I fix this?