This is just one more piece of the puzzle in Leopard. Because /etc/X11 is gone in Leopard, the old way of configuring is obsolete. The new way is org.x.X11_launcher.plist. So the following,

defaults write org.x.X11_launcher app_to_run /usr/X11/bin/xlsclients

is the way to configure to not start an xterm every time it starts.

UPDATE: changed from xhost to xlsclients on the recommendation of this useful page (see the second page).