static void
possibly_show_fallback_dialog (GsmManager *manager)
{
        if (manager->priv->is_fallback_session &&
            strcmp (manager->priv->session_name, "gnome-fallback") == 0 &&
            g_settings_get_boolean (manager->priv->settings,
                                    KEY_SHOW_FALLBACK_WARNING)) {
...
        } else if (strcmp (manager->priv->session_name, "gnome") == 0 &&
...
        }

This assumes that the session name is always set.

However:

gboolean
gsm_session_fill (GsmManager  *manager,
                  char       **override_autostart_dirs,
                  const char  *session)
{
        GKeyFile *keyfile;
        gboolean is_fallback;
        char *actual_session;

        if (override_autostart_dirs != NULL) {
                load_override_apps (manager, override_autostart_dirs);
                return TRUE;
        }

        if (IS_STRING_EMPTY (session))
                session = _gsm_manager_get_default_session (manager);

If we pass autostart overrides, setting of the session name, etc. doesn't happen.

Hence, the gdm greeter (which passes overrides) goes into a crash loop.

gnome-session-2.91.90-1.fc15.1.x86_64
gdm-2.91.6-10.fc15.x86_64

Not being a core dev, dunno whether the fix is to initialize the session name, etc. in all cases, or to switch GDM to defining a session that it passes in.