Here is the error I get when I am trying to install the updates suggested by the Update Manager in my Ubuntu 11.10. I have not installed and/or removed gnome/KDE/xfce or any other window manager. It is as it was at the time of installation.
Probably some package removed it. If you use ubuntu (gnome), execute these commands:
This will install the meta-packages and perform a full upgrade (will not change ubuntu version). Reboot your computer.