GNOME with Xmonad session requires gnome-panel
After installing the xmonad package, the login session "GNOME with Xmonad" is offered, but it cannot start, instead issuing an error message (from memory) "unable to open 'xmonad'".
This seems to be caused by the absence of the gnome-panel package; installing this makes things work :-)
I'm not quite sure how I ended up without gnome-panel as I upgraded to 12.04, but it was absent. Perhaps the package dependencies for Xmonad should require this? Not necessarily an easy decision however, but some indication of the dependency is needed.