Index: protected_node.module
===================================================================
--- protected_node.module	(revision 4991)
+++ protected_node.module	(working copy)
@@ -340,3 +337,23 @@
 
   return $form_element;
 }
+
+/**
+ * Prevent boost from caching protected nodes.
+ *
+ * We would also need to make sure the cache is cleared whenever
+ * the protection is turned on.
+ */
+function protected_node_boost_is_cacheable($path) {
+  // the $path may be an alias, unalias first
+  $url = drupal_lookup_path('source', $path);
+  if (!$url) {
+    // $path is not an alias, use $path as is for our test
+    $url = $path;
+  }
+  $p = explode('/', $url);
+  if (count($p) == 2 && $p[0] == 'node' && is_numeric($p[1])) {
+    // if protected, do not cache (i.e. not caching == return FALSE)
+    return !protected_node_isset_protected($p[1]);
+  }
+}