I am starting to worry that Ubuntu is increasingly messing up things and will hate to see Canonical going down the path. In my Ubuntu Server (11.10 Oneiric Ocelot), ethernet broke down as I updated the Unity interface. The DNS does not work. resolv.conf file was empty.
After spending quite sometime finally figured that the Network Manager configuration is turned false. So, if you see your network connection showing up as unmanaged, follow these steps (as sudo, of course):
Edit /etc/NetworkManager/NetworkManager.conf :
change managed=false to managed=true
Then restart the network as follows
# service network-manager stop
# service network-manager start
Thought this may be of use to someone.