Index: wp-settings.php
===================================================================
--- wp-settings.php	(revision 12605)
+++ wp-settings.php	(working copy)
@@ -323,6 +323,20 @@
 	return false;
 }
 
+/**
+ * Whether Multisite support is enabled
+ * 
+ * @since 3.0
+ *
+ * @return bool True if multisite is enabled, false otherwise.
+ */
+function is_multisite() {
+	if ( defined('MULTISITE') )
+		return true;
+
+	return false;
+}
+
 if ( file_exists(WP_CONTENT_DIR . '/object-cache.php') ) {
 	require_once (WP_CONTENT_DIR . '/object-cache.php');
 	$_wp_using_ext_object_cache = true;
@@ -388,6 +402,11 @@
 require (ABSPATH . WPINC . '/http.php');
 require (ABSPATH . WPINC . '/widgets.php');
 
+if ( is_multisite() ) {
+	require_once( ABSPATH . WPINC . '/ms-functions.php' );
+	require_once( ABSPATH . WPINC . '/ms-default-filters.php' );
+}
+
 if ( !defined('WP_CONTENT_URL') )
 	define( 'WP_CONTENT_URL', get_option('siteurl') . '/wp-content'); // full url - WP_CONTENT_DIR is defined further up
 
Index: wp-admin/ms-edit.php
===================================================================
--- wp-admin/ms-edit.php	(revision 12605)
+++ wp-admin/ms-edit.php	(working copy)
@@ -1,5 +1,9 @@
 <?php
 require_once('admin.php');
+
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 if( is_site_admin() == false ) {
 	wp_die( __('You do not have permission to access this page.') );
 }
Index: wp-admin/ms-sites.php
===================================================================
--- wp-admin/ms-sites.php	(revision 12605)
+++ wp-admin/ms-sites.php	(working copy)
@@ -1,6 +1,9 @@
 <?php
 require_once('admin.php');
 
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 $title = __('WordPress MU &rsaquo; Admin &rsaquo; Blogs');
 $parent_file = 'wpmu-admin.php';
 
Index: wp-admin/ms-upgrade-site.php
===================================================================
--- wp-admin/ms-upgrade-site.php	(revision 12605)
+++ wp-admin/ms-upgrade-site.php	(working copy)
@@ -1,12 +1,16 @@
 <?php
 require_once('admin.php');
+
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 require_once( ABSPATH . WPINC . '/http.php' );
 
 $title = __('WordPress MU &rsaquo; Admin &rsaquo; Upgrade Site');
 $parent_file = 'wpmu-admin.php';
 require_once('admin-header.php');
 
-if( is_site_admin() == false ) {
+if ( is_site_admin() == false ) {
     wp_die( __('You do not have permission to access this page.') );
 }
 
Index: wp-admin/ms-users.php
===================================================================
--- wp-admin/ms-users.php	(revision 12605)
+++ wp-admin/ms-users.php	(working copy)
@@ -1,6 +1,9 @@
 <?php
 require_once('admin.php');
 
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 $title = __('WordPress MU &rsaquo; Admin &rsaquo; Users');
 $parent_file = 'wpmu-admin.php';
 
Index: wp-admin/ms-admin.php
===================================================================
--- wp-admin/ms-admin.php	(revision 12605)
+++ wp-admin/ms-admin.php	(working copy)
@@ -1,6 +1,9 @@
 <?php
 require_once('admin.php');
 
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 $title = __('WordPress MU &rsaquo; Admin');
 $parent_file = 'wpmu-admin.php';
 
@@ -11,7 +14,7 @@
 
 require_once('admin-header.php');
 
-if( is_site_admin() == false ) {
+if ( is_site_admin() == false ) {
 	wp_die( __('You do not have permission to access this page.') );
 }
 
Index: wp-admin/ms-options.php
===================================================================
--- wp-admin/ms-options.php	(revision 12605)
+++ wp-admin/ms-options.php	(working copy)
@@ -1,5 +1,9 @@
 <?php
 require_once('admin.php');
+
+if ( !is_multisite() )
+	wp_die( __('Multisite support is not enabled.') );
+
 $title = __('WordPress MU &rsaquo; Admin &rsaquo; Site Options');
 $parent_file = 'wpmu-admin.php';