When XMonad was used with GNOME, through EwmhDesktops and the last window was destroyed, it would not be deleted from the window list and from the workspace switcher. This bug report explains the situation. To correct it, I’ve made a patch to libwnck’s screen.c, which was applied in the SVN version of this file. I’ve done a package for Ubuntu Hardy Heron and other for Debian Etch with a corrected version of this library.