Here is an easy way to use Cairo-Dock as a shell, instead of Unity on Ubuntu Oneiric (11.10) and newer.
It will remove the top panel and the left bar.

• Log-out from your session
• Click on the small Ubuntu button just next to the password entry; it will pop up a menu with the different available sessions. There should be Ubuntu, Ubuntu2D, Cairo-dock and Cairo-Dock without effect
• Choose the Cairo-Dock session (or the one without the effects if your graphic card doesn’t support them)
• Enter your password and press enter

Now, the next time you’ll start your PC, you will get into the cairo-dock session.
Notes:

• Note: if you notice that alt-tab is not working any more, or anything else, open ccsm, and enable the extensions you need or instance, alt-tab is handled by the Shift-switcher extension or the Application switcher extension. If you don’t have them, just install the package compiz-plugins. Also don’t forget to enable the Dbus plug-in in ccsm to allow the dock to interact with Compiz.
• Note¹: You may want to use a more complete theme than the default one: Right-click on the dock -> Cairo-Dock -> Configure -> Theme tab and choose the Oneiric-Panel or Unity-1 theme. The first one is a theme like a Gnome2 theme with a dock and the other one is a theme that is quite similar to Unity (left dock with launchers and taskbar, top panel with several useful applets).
• Note²: the first time you log into the cairo-dock session, it may take a few seconds to start. This is because Unity is being removed. The next time, it will be much faster
• Note³: Unity is not deleted, it will only not be launched, so you will not break anything. You can revert the changes by selecting the Ubuntu session during the log-in.
• Note⁴: If your session is automatically launched (if you don’t have to set your password at startup to log in), it’s possible that the Unity session is launched instead of our Cairo-Dock session. If you want to force the Cairo-Dock session, simply edit this file with root right: /etc/lightdm/lightdm.conf, e.g. with this command in a terminal:
  sudo gedit /etc/lightdm/lightdm.conf

  And replace this line:

  user-session=ubuntu

  by this one

  user-session=cairo-dock