Bug#354458: Kernel panic after upgrading initramfs-tools
OK, I got a normal boot again, by upgrading udev to 0.087-2 (from
unstable). It appears that the initramfs-tools from unstable also needs
a newer udev. This does make the ide-disk module get loaded and the
machine boots normally now. Of course I still don't know what the
"can't find init" problem was, but upgrading udev and initramfs-tools
(both!) to the ones currently in unstable certainly solved that.
(Case closed, for now!)