Comment 8 for bug 855171

Same problem here, although I was only playing with installing the gnome-shell package at the time. Because of the error, the "network" option in the gnome-control-panel ceased working (I'm using GNOME3) and the network-manager-applet (if this is still its name in GNOME3) disappeared. Had to add the classic eth0 line in my /etc/network/interfaces and connect through ethernet cable to get it working:
$ sudo vim /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
esc :wq
$ sudo ifup eth0

(quoted a bit roughly but maybe that'll help newbies fix the problem temporarily)