This was on a freshly installed and dist-updated i386 Ubuntu 12.10 Quantal Quetzal .
apt-get install --no-install-recommends xmonad libghc-xmonad-contrib-dev gnome-panel indicator-applet-complete indicator-multiload
indicator-multiload is optional, but I like it.
import XMonad import XMonad.Config.Gnome main=xmonad gnomeConfig
Logout, the choose the "GNOME with Xmonad" login option available above and to the right of the password field. indicator-multiload fails to start on its own at first. Run it from the command line. It will start correctly, automatically, on future logins.
These instructions also seem to work on Ubuntu 13.04 Raring Ringtail i386.