Thank you all ! adding the --partition 11 to mkofboot made it.
In case someone has the same problem, the whole command was : mkofboot -b /dev/hda9 -m /usr/lib/yaboot/ofboot --root /dev/hda11 --partition 11 for a Apple_Boostrap partition on hda9 and / on hda11 (hda10 is swap). I had no time to try giving hd:11,/boot/vmlinux-2.2.18pre21 to yaboot prompt a try, as suggested by Wilhelm Fitzpatrick, because Ethan's solution worked (I would have been surprised it didn't :-) Thank you one more time, Franck
