Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 18316)
+++ wp-includes/load.php	(working copy)
@@ -639,9 +639,12 @@
 	if ( defined( 'MULTISITE' ) )
 		return MULTISITE;
 
-	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )
+	if ( is_subdomain_install() )
 		return true;
 
+	if ( defined( 'SUNRISE' ) )
+		return true;
+
 	return false;
 }
 
Index: wp-includes/ms-load.php
===================================================================
--- wp-includes/ms-load.php	(revision 18316)
+++ wp-includes/ms-load.php	(working copy)
@@ -16,7 +16,7 @@
  * @return bool True if subdomain configuration is enabled, false otherwise.
  */
 function is_subdomain_install() {
-	if ( defined('SUBDOMAIN_INSTALL') )
+	if ( defined('SUBDOMAIN_INSTALL') && SUBDOMAIN_INSTALL == true)
 		return SUBDOMAIN_INSTALL;
 
 	if ( defined('VHOST') && VHOST == 'yes' )