[Removing GNOME3.0 target as decided in release-team meeting on March 03, 2011. "nice-to-have" categorisation for GNOME3.0]

So there are a few problems with a separate key

1) It's not just one separate key, but is 7 that together define how the background is rendered.

2) gdm doesn't draw the background itself, gnome-settings-daemon does.  gnome-settings-daemon doesn't run in a "GDM mode" or anything so it wouldn't know to use the 7 different keys (and it uses the key through an abstracted api in gnome-desktop, anyway, so we'd need new api to tell gnome-desktop to use the new keys)

But, it turns out the screensaver and the login screen already use the same background, though, because screensaver has this code:

gs-manager.c-        /* FIXME: we need to bind system settings instead of user but
gs-manager.c-         *        that's currently impossible, not implemented yet.
gs-manager.c-         *        Hence, reset to system default values.
gs-manager.c-         */
gs-manager.c-        /* TODO: Ideally we would like to bind some other key, screensaver-specific. */
gs-manager.c-        settings = g_settings_new ("org.gnome.desktop.background");
gs-manager.c-
gs-manager.c:        g_settings_delay (settings);
gs-manager.c-
gs-manager.c-        keys = g_settings_list_keys (settings);
gs-manager.c-        for (k = keys; *k; k++) {
gs-manager.c-                g_settings_reset (settings, *k);
gs-manager.c-        }
gs-manager.c-        g_strfreev (keys);

the g_settings_delay() call means no changes to org.gnome.desktop.background will get propagated to the users session, and then the rest of the code forces schema defaults (or the administrator overrides).

So we're actually good enough for now, I think.  It would be good if we had a better UI story here for admins, and indeed a more fleshed out admin story overall in gsettings/dconf world, but those are separate issues.

*** Bug 645073 has been marked as a duplicate of this bug. ***