Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.777
diff -u -r1.777 system.module
--- modules/system/system.module	26 Aug 2009 10:53:45 -0000	1.777
+++ modules/system/system.module	27 Aug 2009 21:57:36 -0000
@@ -469,7 +469,6 @@
   $type['fieldset'] = array(
     '#collapsible' => FALSE,
     '#collapsed' => FALSE,
-    '#value' => NULL,
     '#process' => array('form_process_fieldset', 'ajax_process_form'),
     '#pre_render' => array('form_pre_render_fieldset'),
     '#theme_wrappers' => array('fieldset'),
Index: includes/ajax.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/ajax.inc,v
retrieving revision 1.7
diff -u -r1.7 ajax.inc
--- includes/ajax.inc	27 Aug 2009 04:40:12 -0000	1.7
+++ includes/ajax.inc	27 Aug 2009 21:57:33 -0000
@@ -186,6 +186,9 @@
   // Since some of the submit handlers are run, redirects need to be disabled.
   $form['#redirect'] = FALSE;
 
+  // Use the validate section handling on the clicked button (if available).
+  $form['#validate_section_allowed'] = TRUE;
+
   // The form needs to be processed; prepare for that by setting a few internal
   // variables.
   $form_state['input'] = $_POST;
Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.367
diff -u -r1.367 form.inc
--- includes/form.inc	27 Aug 2009 04:40:12 -0000	1.367
+++ includes/form.inc	27 Aug 2009 21:57:34 -0000
@@ -705,13 +705,27 @@
   // If the session token was set by drupal_prepare_form(), ensure that it
   // matches the current user's session.
   if (isset($form['#token'])) {
-    if (!drupal_valid_token($form_state['values']['form_token'], $form['#token'])) {
+    if (!drupal_valid_token($form['form_token']['#value'], $form['#token'])) {
       // Setting this error will cause the form to fail validation.
       form_set_error('form_token', t('Validation error, please try again. If this error persists, please contact the site administrator.'));
     }
   }
 
-  _form_validate($form, $form_state, $form_id);
+  // Allow only a portion of the form to be validated. This is only allowed if
+  // the #validate_section property is on a button AND a custom #submit property
+  // is defined. This prevents the form-level submit handlers from firing
+  // accidentally when a portion of the form has not been validated.
+  $elements = $form;
+  $button = isset($form_state['clicked_button']) ? $form_state['clicked_button'] : FALSE;
+  if ($button && !empty($form['#validate_section_allowed']) && isset($button['#validate_section']) && isset($button['#submit'])) {
+    foreach ($button['#validate_section'] as $section) {
+      $elements = $elements[$section];
+    }
+  }
+
+  // Validate the section of the form.
+  _form_validate($elements, $form_state, $form_id);
+
   $validated_forms[$form_id] = TRUE;
 }
 
@@ -768,7 +782,8 @@
  *   not be repeated in the submission step.
  * @param $form_id
  *   A unique string identifying the form for validation, submission,
- *   theming, and hook_form_alter functions.
+ *   theming, and hook_form_alter functions. Indicates that we are starting a
+ *   new validate operation.
  */
 function _form_validate($elements, &$form_state, $form_id = NULL) {
   // Also used in the installer, pre-database setup.
@@ -780,6 +795,12 @@
       _form_validate($elements[$key], $form_state);
     }
   }
+
+  // Only validated elements receive a value.
+  if (array_key_exists('#value', $elements)) {
+    form_set_value($elements, $elements['#value'], $form_state);
+  }
+
   // Validate the current input.
   if (!isset($elements['#validated']) || !$elements['#validated']) {
     if (isset($elements['#needs_validation'])) {
@@ -827,7 +848,7 @@
     // #value data.
     elseif (isset($elements['#element_validate'])) {
       foreach ($elements['#element_validate'] as $function) {
-        if (function_exists($function))  {
+        if (function_exists($function) && $function != '_form_validate')  {
           $function($elements, $form_state, $form_state['complete form']);
         }
       }
@@ -1184,7 +1205,6 @@
       }
     }
   }
-  form_set_value($element, $element['#value'], $form_state);
 }
 
 /**