Comment 5 for bug 984276

I think why this is only happening for some people is that update-initramfs doesn't end up being a symlink for them. casper.postinst contains the following:

#! /bin/sh

# If running from writable media, make sure that update-initramfs is
# properly a symlink.
if [ "$1" = configure ] && dpkg --compare-versions "$2" lt-nl 1.253 && \
   [ -w /cdrom ] && [ -e /usr/sbin/update-initramfs.distrib ] && \
   [ ! -L /usr/sbin/update-initramfs ] && \
   fgrep -qs update-initramfs.distrib /usr/sbin/update-initramfs; then
    ln -nsf /usr/share/casper/casper-update-initramfs \
            /usr/sbin/update-initramfs
fi

update-initramfs -u

#DEBHELPER#

If you just install casper and don't pass all the tests update-initramfs won't be symlinked to casper-update-initramfs and then you will be able to install kernel updates. However, if the symlink is created you'll likely have problems using update-initramfs.