In RHEL4 and possibly other distros. The .Xdefaults file is not used, the file is now called .Xresources
In /etc/X11/xinit/xinitrc-common, xrdb is passed .Xresources _after_ /etc/X11/Xresources.
You can make systemwide changes in /etc/X11/Xresources, but they will be overridden by changes to .Xresources.
