Index: wp-includes/default-constants.php
===================================================================
--- wp-includes/default-constants.php	(revision 17644)
+++ wp-includes/default-constants.php	(working copy)
@@ -64,6 +64,10 @@
 
 	if ( !defined('SHORTINIT') )
 		define('SHORTINIT', false);
+
+	if ( !defined( 'SINGLESITE' ) )
+		define( 'SINGLESITE', ! defined( 'MULTISITE' ) || ! MULTISITE );
+
 }
 
 /**
Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 17644)
+++ wp-includes/load.php	(working copy)
@@ -622,6 +622,9 @@
  * @return bool True if multisite is enabled, false otherwise.
  */
 function is_multisite() {
+	if ( defined( 'SINGLESITE' ) )
+		return SINGLESITE;
+
 	if ( defined( 'MULTISITE' ) )
 		return MULTISITE;
 
Index: wp-settings.php
===================================================================
--- wp-settings.php	(revision 17644)
+++ wp-settings.php	(working copy)
@@ -13,6 +13,7 @@
  *
  * @since 1.0.0
  */
+
 define( 'WPINC', 'wp-includes' );
 
 // Include files required for initialization.
@@ -86,8 +87,10 @@
 if ( is_multisite() ) {
 	require( ABSPATH . WPINC . '/ms-blogs.php' );
 	require( ABSPATH . WPINC . '/ms-settings.php' );
-} elseif ( ! defined( 'MULTISITE' ) ) {
+} elseif ( SINGLESITE || ! defined( 'MULTISITE' ) ) {
 	define( 'MULTISITE', false );
+	if ( defined( 'SUNRISE' ) )
+		include_once( WP_CONTENT_DIR . '/sunrise.php' );
 }
 
 // Stop most of WordPress from being loaded if we just want the basics.