Comment 2 for bug 797509

This thing might also be useful:

napster@panther ~ $ sudo apt-get install -f gnome3-session
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 gnome3-session : Depends: gnome-shell but it is not installable
E: Broken packages

I guess gnome-shell repo is not enabled by default and gnome3-session repo is enabled, which causes this error!