Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.343
diff -u -r1.343 form.inc
--- includes/form.inc	20 Jun 2009 15:17:38 -0000	1.343
+++ includes/form.inc	25 Jun 2009 08:29:17 -0000
@@ -2481,8 +2481,12 @@
  * @ingroup themeable
  */
 function theme_form($element) {
+  // PHP drops the $_POST array if this exceeds the PHP setting post_max_size.
+  $action = '';
+  if ($element['#action']) {
+    $action = 'action="' . check_url($element['#action']) . (strpos($element['#action'], '?') !== FALSE ? '&' : '?') . 'form-id=' . check_plain($element['#build_id']) . '"';
+  }
   // Anonymous div to satisfy XHTML compliance.
-  $action = $element['#action'] ? 'action="' . check_url($element['#action']) . '" ' : '';
   return '<form ' . $action . ' accept-charset="UTF-8" method="' . $element['#method'] . '" id="' . $element['#id'] . '"' . drupal_attributes($element['#attributes']) . ">\n<div>" . $element['#children'] . "\n</div></form>\n";
 }