Kernel panic: No init found?

["Guy T. Rice" <guyrice@gmail.com>]

I've tried installing sarge on a SPARCstation 20 a few times now,
using the r0a CD image.  The installer works fine until you go to
reboot the system, but when you reboot, the kernel panics with this
error:

Kernel panic: No init found. Try passing init= option to kernel

I've tried init=/sbin/init and init=/bin/sh, neither of which works,
despite the fact that if I look either at the root filesystem or the
initrd filesystem (after loopback mounting
/boot/initrd.img-2.4.27-2-smp) these files are present.

Figuring that it might be an error in the initrd.img, I pulled one of
the processors out of the machine to cause d-i to install a different
kernel.  That fixed the problem.  However, now I'm not using an SMP
kernel.  If I switch back to the SMP kernel and the problem reappears,
is there some way to fix this short of reverting to the UP kernel?