=== modified file 'includes/module.inc'
--- includes/module.inc	2007-04-17 07:19:38 +0000
+++ includes/module.inc	2007-04-30 22:39:45 +0000
@@ -341,10 +341,24 @@ function module_implements($hook, $sort 
  *   The return value of the hook implementation.
  */
 function module_invoke() {
+  static $includes;
   $args = func_get_args();
   $module = array_shift($args);
   $hook = array_shift($args);
   $function = $module .'_'. $hook;
+  if (!isset($includes[$module][$hook])) {
+    $underscore = strpos($hook, '_');
+    if ($underscore !== FALSE) {
+      $include = substr($hook, 0, $underscore);
+      $filename = dirname(drupal_get_filename('module', $module)) . "/$include.inc";
+      $included = FALSE;
+      if (file_exists($filename)) {
+        include_once($filename);
+        $included = TRUE;
+      }
+      $includes[$module][$hook] = $included;
+    }
+  }
   if (module_hook($module, $hook)) {
     return call_user_func_array($function, $args);
   }