Comment 3 for bug 19686

there is an upstream bug: "Shortcut for logging out"
http://bugzilla.gnome.org/show_bug.cgi?id=99335

which is actually a debate about "ctrl-alt-del" vs "alt-f4" to do this.

According to the Gnome's Human Interface Guidelines, Ctrl-alt-del are the keys
reserved for "Open the session logout confirmation dialog"
http://developer.gnome.org/projects/gup/hig/2.0/input-keyboard.html#standard-shortcuts