The shortcut editor accepts modifiers (Shift, Ctrl, Atl, ...) as accelerators, but this does not seem to be supported by xfwm4, so the shortcut editor should not accept them or atleast warn the user about this.
fixing #378 seems to have fixed this one
There's still possible to add shortcut to example Shift+Mode_switch (windows menu key for me) but xfwm4 doesn't support it? (for 4.2beta2)
Super_L is seen as Mod4
should be fixed in CVS now
See that thread for more details : http://foo-projects.org/pipermail/xfce4-dev/2006-April/019880.html It's adk/gdk issue that should be fixed in gtk+ 2.10