Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 15155)
+++ wp-includes/ms-functions.php	(working copy)
@@ -454,6 +454,8 @@
 function wpmu_validate_user_signup($user_name, $user_email) {
 	global $wpdb;
 
+	require_once( ABSPATH . WPINC . '/registration.php' );
+
 	$errors = new WP_Error();
 
 	$orig_username = $user_name;
@@ -546,6 +548,8 @@
 function wpmu_validate_blog_signup($blogname, $blog_title, $user = '') {
 	global $wpdb, $domain, $base, $current_site;
 
+	require_once( ABSPATH . WPINC . '/registration.php' );
+
 	$blog_title = strip_tags( $blog_title );
 	$blog_title = substr( $blog_title, 0, 50 );
 
@@ -720,6 +724,8 @@
 function wpmu_activate_signup($key) {
 	global $wpdb, $current_site;
 
+	require_once( ABSPATH . WPINC . '/registration.php' );
+
 	$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE activation_key = %s", $key) );
 
 	if ( empty($signup) )