Since I upgraded Gnome from 2.24 to something more recent, what was then Ctrl+Alt+Del (or <Control><Alt>Delete speaking of gconf settings) didn’t show the logout dialog anymore, but rather the shutdown/reboot one! I like to be able to log out or switch user with a keyboard shortcut, however, there is no predefined action which would lead to that dialog anymore.

As a workaround, create the following entries in the gnome registry, for example using gconf-editor:
in /apps/metacity/global_keybindings/run_command_10, enter the keybinding (e.g. <Control><Alt>Delete)
in /apps/metacity/keybinding_commands/command_10, use the command gnome-session-save --logout-dialog --gui — this will do the magic trick of opening the log out dialog.

Disable the old binding to shutdown using gnome-keybinding-properties by clicking on the entry and hitting backspace, if desired.

People are claiming that xmessage does not support unicode utf-8, or that you should switch to some gnome crap instead. Try specifying the resource *international: true and it will work as advertised. (You might still have to set up your fonts properly.)

By the way, a proper Gnome-alternative is Zenity.