Comment 27 for bug 139368

As suggested by Nick and several others, I've installed dbus-x11. It may be awhile before I know if this fixes my symptom reported above, but it looks like there is a bug in the package dependency declarations anyway. When the dbus package was split into dbus and dbus-x11, some other packages (gnome-session seems like a likely candidate) should have had their depends-on link to the dbus package changed to the dbus-x11 package, but didn't.