Index: includes/module.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/module.inc,v
retrieving revision 1.158
diff -u -p -r1.158 module.inc
--- includes/module.inc	27 Sep 2009 11:08:45 -0000	1.158
+++ includes/module.inc	29 Sep 2009 08:14:18 -0000
@@ -421,7 +421,7 @@ function module_implements_write_cache()
   // cacheable for other reasons than the HTTP request type as well, but this
   // does not matter for the hook implementation cache, because nothing breaks
   // if hook implementations are not cached.
-  if (isset($implementations['#write_cache']) && drupal_page_is_cacheable()) {
+  if (isset($implementations['#write_cache']) && ($_SERVER['REQUEST_METHOD'] == 'GET' || $_SERVER['REQUEST_METHOD'] == 'HEAD')) {
     unset($implementations['#write_cache']);
     cache_set('module_implements', $implementations);
   }