As X mode is known to have problems sometimes, it would be nice if xdm could ask confirmation before starting, it would be easier to make things work.
Created attachment 25767 [details, diff] patch against /usr/portage/x11-base/xfree/files/4.3.0/xdm.start This will make /etc/init.d/xdm ask for confirmation before starting. There is a timeout of 5 seconds after what xdm will start anyway (working this way the same as before).
Even if it tries to start and fails, you should get a reasonable error message. Also this has been largely preempted by the newer baselayout's interactive boot options and the nox boot-time option.