Here are my notes on a successful kernel build/boot.
I hope this helps others but use this at your own risk
powerbook with standard ydl6.1 installation
downloaded linux-2.6.23.9.tar.bz2
from
http://www.kernel.org/pub/linux/kernel/v2.6/this is the same version as ydl 6.1
bunzip2, tar xvf, cd linux-2.6.23.9
make menuconfig, and save the config file to .config
(the .config differs from ydl's but I have no idea what that means - view with diff or diffuse)
make
make a backup of /lib/modules subtree using tar czf
make modules_install per build instructions (it added new dir: /lib/modules/2.6.23.9)
make backup copy of /boot (cp -R /boot /bootbak)
cp vmlinux /boot/vmlinux-2.6.23.9.custom
cp System.map /boot/System.map-2.6.23-9.custom
cp .config /boot/config-2.6.23.9.custom
add this to bottom of /boot/etc/yaboot.conf :
image=/vmlinux-2.6.23-9.custom
label=custom
read-only
initrd=/initrd-2.6.23-9.ydl6.1.img
append="rhgb quiet root=LABEL=/1"
ybin -v is required to "register" the above changes.
reboot and at yaboot's second promp use tab key to see the two options - ydl6.1 and custom
both boot
Now you can experiment with the kernel.
I'm going to look into radeon_backlight.c and see what's causing the backlight to flash when I turn it off.