Yeah, I can't get grub to install on my 15" PB 1.33 GHz. I went back, and followed steps 4-7 again, yet no dice.
Same experience on my DLSD. A warning notice was placed at the beginning for a reason.
There's an active problem with GRUB going on where it gets the wrong ofpath names, and its boot orders are screwed up, and as a result, won't install properly. If you already happen to have Sid installed, and update GRUB to the newest version, you'll essentially get locked out and will need to resort to OF to get back in. To top that, Yaboot doesn't want to install as a fallback for whatever reason.
My advice is to either wait for the fix, or just install Wheezy, which is the last version with next to no issues with PowerPC machines (in my experience). Jessie screwed some things up with the kernel images, and had some goofy nouveau issues, which shouldn't be too much of a problem on G4s, but basically borks the experience on G5s. It can be fixed with modified kernel images available off the web, but that takes a slight amount of know-how to pull off. [EDIT: Not anymore! Visit The Linux Thread for details.]
Another option, download one of the older images, like 04-09, and use the archived guide to install that. That should work OK.
In any case, my DLSD is now happily running Wheezy, the browser situation isn't bad, sound was easy to get working, the trackpad is working fine out of the box, and, *gasp*, it goes to and wakes from sleep with flying colors, which is something I don't think we'll ever see past kernel version 3.2.
EDIT: I can confirm Arctic Fox is working under 7.11 after installing a couple required packages.
I'll make a thread with a bunch of fixes, tweaks, and workarounds to get Ubuntu and Debian versions (not exclusive to Sid) running as well as possible sometime soon. I'm actually kind of looking forward to it. These older versions are still pretty good, and are still far more secure and updated than Leopard.