"debug" level was supposed to be hidden, since it's just an alias for debug2.

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.459 2008/07/01 06:08:31 mha Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.460 2008/07/01 06:36:11 mha Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -178,7 +178,7 @@ static char *config_enum_get_options(struct config_enum *record,
  * they sort slightly different (see "log" level)
  */
 static const struct config_enum_entry client_message_level_options[] = {
-	{"debug", DEBUG2, false},
+	{"debug", DEBUG2, true},
 	{"debug5", DEBUG5, false},
 	{"debug4", DEBUG4, false},
 	{"debug3", DEBUG3, false},
@@ -195,7 +195,7 @@ static const struct config_enum_entry client_message_level_options[] = {
 };
 
 static const struct config_enum_entry server_message_level_options[] = {
-	{"debug", DEBUG2, false},
+	{"debug", DEBUG2, true},
 	{"debug5", DEBUG5, false},
 	{"debug4", DEBUG4, false},
 	{"debug3", DEBUG3, false},