Index: wp-includes/js/tinymce/tiny_mce_config.php
===================================================================
--- wp-includes/js/tinymce/tiny_mce_config.php	(revision 7147)
+++ wp-includes/js/tinymce/tiny_mce_config.php	(working copy)
@@ -155,7 +155,7 @@
 // Cache path, this is where the .gz files will be stored
 $cache_path = ABSPATH . 'wp-content/uploads/js_cache'; 
 if ( $disk_cache && ! is_dir($cache_path) )
-	$disk_cache = mkdir($cache_path);
+	$disk_cache = wp_mkdir_p($cache_path);
 
 $cache_ext = '.js';