I installed xmonad a couple of nights ago. Xmonad includes the files that you would need select it from GDM (e.g. /usr/share/applications/xmonad.desktop and /usr/share/xsessions/xmonad.desktop).
I tried starting xmonad via gdm and locked up X... (eventually even the virtual terminals locked up and I had to reboot the machine). The next time I started the machine I selected gnome instead of xmonad, it launched gnome-session, but tried to run xmonad instead of metacity as the desktop environment (this was entirely unusable). I moved the xmonad.desktop files in both /usr/share/applications and /usr/share/xsessions, now I have no desktop environment (which is slightly usable). How do I re-enable metacity?