Jeff said: 
Yes -- there has been an outstanding ticket for
"research security-relevant gconf settings" for quite a while:

I twigged to this by looking at Steve Grubb's kickstart scripts for securing at install, and wondered where this group was. 
 
If you'd like to take stab at it, patches are welcome!

lol, we'll see after I get a chance to finish my server install to test the project. 

And Maura and Shawn pointed out ways to turn this off
GDM by default will read from the gconf settings folder in
the gdm user's home directory (/var/lib/gdm/.gconf).

gconftool-2 -s /apps/gdm/simple-greeter/disable_user_list true

I implemented something that I'm not sure is sufficient:
 gconftool-2 --direct --config-source=xml:readwrite:/etc/gconf/gconf.xml.mandatory \
  -t bool -s /apps/gdm/simple-greeter/disable_user_list true

Testing is the easier way, but I would think that the gdm user's .gconf has already been populated at creation, and therefore misses this setting that occurs after installation?
Where is the right place to put changes like these? Should I be changing /var/lib/gdm/.gconf?

Andrew