The yaboot binary in Debian is broken.
I took the one from the Fedora package "yaboot-1.3.14-27.fc12.ppc.rpm"

To netboot a PPC mac, normal method is to hold N while booting.
But "boot enet:" doesn't work, you need to specify a filename
to get the mac to talk non-apple DHCP.  And slashes in the filename
seem bad.  So:

- get into OF by booting with option-apple-o-f
- run "boot enet:,yaboot"