I also have this annoying bug. Gandalf resolve it in is compiz repo by patching gnome-session. I attach the actual patch for the 2.16 version.
** Attachment added: "gnome-session_patch" http://librarian.launchpad.net/4441152/19_logout_no_grab_server.patch -- GNOME logout screen doesn't appear when the composite is enabled https://launchpad.net/bugs/24221 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
