It's really easy with Mate or gnome-flashback.

  sudo apt-get install mate-desktop

gets you a really nice desktop, almost identical to Ubuntu pre-Unity. Then you just need to use

  main = xmonad mateConfig

in xmonad.hs and change the window manager using gconf-editor.

Unity 3D is a Compiz plugin so you can't use XMonad with it.

Gnome 3.8 and later only support gnome-shell as a window manager; it is not possible to integrate xmonad or any other window manager with it