This sort of thing has been known to happen once or twice.
Here is a fairly good explanation of what might have gone wrong:http://www.dedoimedo.com/computers/ubuntu-initrd-bug.html
This write-up, though it focuses on Ubuntu and PC hardware, is in large part applicable to PowerPC Linux as well, because while PowerPC uses BootX or Yaboot (definitely yaboot in your case) instead of grub or grub2, most of the rest of the possible problems, and their solutions, are similar. In other words, from the moment the boot-loader takes over, Linux behaves similarly, regardless of the underlying hardware.
I'd focus first on the boot arguments, about here:
Boot arguments, which you can see by running cat /proc/cmdline in the BusyBox shell
Basically, anytime it refers to grub, just substitute yaboot. See also:http://www.alaska.net/~erbenson/yaboot/