Yes, it seems to happen when the xinitrc uses the following command format to start a window manager:
ck-launch-session dbus-launch --exit-with-session <window-manager>
You could try changing /etc/X11/xdm/xdm-config to use
DisplayManager.*.authName: MIT-MAGIC-COOKIE-1 if it's still using XDM-AUTHORISATION (already changed in slackware-current I believe) and see if that helps.
Alternatively, you'll need to use an xinitrc that uses the
eval `dbus-launch --sh-syntax --exit-with-session` approach to launching dbus. The only problem here is that dbus-launch needs to be run within the console-kit session itself so you'll probably need a startwm script to run it and then invoke the window manager.
As an example, I'll attach my generic window-manager startup script and an xinitrc that calls it so you can see what I'm talking about. Feel free to do what you want with them.