=== modified file 'includes/module.inc'
--- includes/module.inc	2010-01-28 13:56:24 +0000
+++ includes/module.inc	2010-01-30 16:21:52 +0000
@@ -580,6 +580,11 @@ function module_implements($hook, $sort 
         $implementations['#write_cache'] = TRUE;
       }
     }
+    // Allow modules to change the weight of specific implementations but avoid
+    // an infinite loop.
+    if ($hook != 'module_implements_alter') {
+      drupal_alter('module_implements', $implementations[$hook], $hook);
+    }
   }
   else {
     foreach ($implementations[$hook] as $module => $group) {

=== modified file 'modules/system/system.api.php'
--- modules/system/system.api.php	2010-01-30 07:59:24 +0000
+++ modules/system/system.api.php	2010-01-30 16:21:26 +0000
@@ -970,6 +970,28 @@ function hook_mail_alter(&$message) {
 }
 
 /**
+ *  Alter the registry of modules implementing a hook.
+ *
+ *  This hook is invoked during module_implements(). A module may implement
+ *  this hook in order to reorder the implementing modules, which are otherwise
+ *  ordered by the module's system weight.
+ *
+ *  @param &$implementations
+ *    An array keyed by the module's name. The value of each item corresponds
+ *    to a $group, which is usually FALSE, unless the implementation is in a
+ *    file named $module.$group.inc.
+ *  @param $hook
+ *    The name of the module hook being implemented.
+ */
+function hook_module_implements_alter(&$implementations, $hook) {
+  if ($hook == 'rdf_mapping') {
+    // Move my_module_rdf_mapping() to the end of the list.
+    unset($implementations['my_module']);
+    $implementations['my_module'] = FALSE;
+  }
+}
+
+/**
  * Alter the information parsed from module and theme .info files
  *
  * This hook is invoked in _system_rebuild_module_data() and in