Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 18502)
+++ wp-includes/ms-functions.php	(working copy)
@@ -687,7 +687,7 @@
 		$mydomain = "$domain";
 		$path = $base.$blogname.'/';
 	}
-	if ( domain_exists($mydomain, $path) )
+	if ( domain_exists($mydomain, $path, $current_site->id) )
 		$errors->add('blogname', __('Sorry, that site already exists!'));
 
 	if ( username_exists( $blogname ) ) {