Those defaults have already been loaded into your machine, so the file is only around so that package upgrades work properly. See Dan's message from a recent thread here: http://mail.gnome.org/archives/gdm-list/2009-May/msg00031.html for instructions on how to disable the user list.
Anybody want to create a patch for the docs/C/gdm.xml file to include the example? I think the manual would be easier to understand if it provided more clear examples like this. Likewise, how do you change the background image? Isn't there a gnome-settings-daemon configuration key you need to change? Would be good if the docs provided more information about common g-s-d keys that users are likely to want to change. Brian