My FreeBSD 4.2 install didn't include an entry for the local X server in /usr/X11R6/lib/X11/xdm/Xservers, so it wouldn't bother to start a local X server, even when I stuck xdm in /etc/rc.local as instructed. Add an entry like this to said file:
:0 FreeBSD local /usr/X11R6/bin/X :0
Where the :0's are the display, "FreeBSD" is a tag for Xresources later (doesn't matter much AFAIK), "local" is the type (local or foreign), and "/usr/X11R6/bin/X" is the path to the server.