Ubuntu 12.04 comes with shiny graphical tool to setup system wide global proxy. But as seen in the picture below there is no way to add exception or ignored host.

To overcome this limitation we have to install dconf Editor. You can install by clicking the previous link or by typing sudo apt-get install dconf-tools in terminal. Please note that though the software is call dconf editor its packaged as dconf-tools.

Once installed open dconf editor and navigate to system -> proxy. From here you can add ignore-hosts as string and as seen in the picture below.

You may need to log out and log in for the changes to take effect… just to avoid any noise!