I've got a Powermac G5, so I'm definitely interested in the source you've used. I'll go for a ppc32 kernel first. Getting Debian on my G5 will already be a huge achievement!
go to kernel.org and get the latest kernel (it seems to be 2.6.11.6 now) you can also patch it if you want 2.6.12, it's up to you...
in the /usr/src/linux-2.6.xx/ directory do : cp arch/ppc64/configs/g5_defconfig .config after this : make oldconfig and then a make xconfig or make gconfig check if you have all your usual options in the config and after this : make-kpkg kernel_image should do it... xavier