Index: wp-includes/registration.php
===================================================================
--- wp-includes/registration.php	(revision 12187)
+++ wp-includes/registration.php	(working copy)
@@ -47,7 +47,7 @@
  * @return bool Whether username given is valid
  */
 function validate_username( $username ) {
-	$sanitized = sanitize_user( $username, true );
+	$sanitized = sanitize_user( $username );
 	$valid = ( $sanitized == $username );
 	return apply_filters( 'validate_username', $valid, $username );
 }
@@ -116,7 +116,7 @@
 		$user_pass = wp_hash_password($user_pass);
 	}
 
-	$user_login = sanitize_user($user_login, true);
+	$user_login = sanitize_user($user_login);
 	$user_login = apply_filters('pre_user_login', $user_login);
 
 	if ( empty($user_nicename) )