Every time I upgrade to a new version of Ubuntu, it breaks my xsession (which runs bear Xmonad). And every time I have to mess around for half an hour to find what needs to be done to get it back. All that needs to be done is to modify the file /usr/share/xsessions/xmonad.desktop to look like this:
[Desktop Entry]
Encoding=UTF-8
Name=XMonad
Comment=Lightweight tiling window manager
Exec=/etc/X11/Xsession
Icon=xmonad.png
Type=XSession
