Can't you just use the prebuilt kernels from : http://people.debian.org/~luther/2.6.14-rc5
I was trying to do that. But now I have to apply the patch from Ben for fan control.
You can naturally do it yourself by building either a ppc32->ppc64 cross compiler, or inside a etch/sid chroot, or something. I personally use a sid chroot (man debootstrap for details).
Good, I'll go for sid chroot. As always, you've been a great help. Eduardo.