Comment 5 for bug 520004

found the reason and the solution on my end:

- first i've installed kernel 2.6.33-020633-generic for testing: unfortunatly dkms failed and update-manager removed all nvidia drivers (might say only desactivate), so i uninstalled this kernel and install "nouveau"

- one reboot later, when i've checked for upgrading packages, an initramfs-tools error appear complaining about that missing initrd.img of 2.6.33 kernel. running dpkg --configure -a did not resolve the problem.
Have to erase it : rm -f /var/lib/initramfs-tools/2.6.33-020633-generic

- run again dpkg --configure -a and that problem is gone.

-update & upgrade work fine again.