=== modified file 'includes/form.inc'
--- includes/form.inc	2009-12-29 20:16:09 +0000
+++ includes/form.inc	2009-12-30 19:48:23 +0000
@@ -879,15 +879,34 @@ function drupal_redirect_form($form_stat
  *   theming, and hook_form_alter functions.
  */
 function _form_validate(&$elements, &$form_state, $form_id = NULL) {
-  // Also used in the installer, pre-database setup.
-  $t = get_t();
-
   // Recurse through all children.
   foreach (element_children($elements) as $key) {
-    if (isset($elements[$key]) && $elements[$key]) {
-      _form_validate($elements[$key], $form_state);
-    }
+    _form_validate($elements[$key], $form_state);
   }
+  // Check whether validation needs to be skipped for this element according to
+  // the #validate_values of the clicked button. We have to recurse into all
+  // form elements, because submitted form values can vary based on #parents and
+  // #parents do not necessarily follow the elements in the form structure.
+  // After the complete form has been validated, the button-level #validate
+  // handlers are executed via form_execute_handlers(), so we must not skip
+  // validation on the top-level.
+  if (!isset($form_id) && _form_validate_skip($elements, $form_state)) {
+    // @TODO: obliterate this hack in D8. This should be a simple return.
+    // Instead, we save the form errors and error messages in the beginning and
+    // restore at the end. This code violates encapsulation and should not be
+    // in core.
+    $skip = TRUE;
+    $form_errors = drupal_static('form_set_error', array());
+    $error_messages = isset($_SESSION['messages']['error']) ? $_SESSION['messages']['error'] : NULL;
+  }
+  else {
+    $skip = FALSE;
+  }
+  
+  
+  // Also used in the installer, pre-database setup.
+  $t = get_t();
+
   // Validate the current input.
   if (!isset($elements['#validated']) || !$elements['#validated']) {
     if (isset($elements['#needs_validation'])) {
@@ -943,7 +962,74 @@ function _form_validate(&$elements, &$fo
       }
     }
     $elements['#validated'] = TRUE;
+    // @TODO: obliterate this hack in D8. This should be a simple return.
+    // Instead, we save the form errors in the beginning and restore at the
+    // end.
+    if ($skip) {
+      $current_form_errors = &drupal_static('form_set_error', array());
+      $current_form_errors = $form_errors;
+      $_SESSION['messages']['error'] = $error_messages;
+    }
+  }
+}
+
+/**
+ * Check if the validation of an element needs to be skipped.
+ *
+ * Multistep forms not wanting to validate the whole form can set the
+ * #validate_values property on buttons to avoid the validation of some
+ * elements. For example, pressing the "Previous" button should not fire
+ * validation errors just because the current step has invalid values. AJAX is
+ * another typical example.
+ *
+ * If this property is set on the clicked button, the form-level #validate and
+ * #submit handlers won't be executed. Only button-level #validate and #submit
+ * handlers will be executed and extreme care should be taken, since
+ * $form_state['values'] will contain not validated values. However, that is
+ * typically not a problem as these buttons (like "Previous" or "Add more") do
+ * not invoke persistent storage of the submitted form values.
+ *
+ * Do not use this property on buttons that trigger saving of form values to the
+ * database.
+ *
+ * The #validate_values property is a list of $form_state['values'] keys. For
+ * example:
+ * @code
+ *   $form['actions']['previous']['#validate_values'] = array(
+ *     array('foo', 'bar'),
+ *     array('step1'),
+ *   );
+ * @endcode
+ * This will validate $form_state['values']['step1']['choice'], since the first
+ * key is 'step1'. Anything under $form_state['values']['step2'] won't be
+ * validated.
+ * $form_state['values']['foo'] won't be validated, but everything under
+ * $form_state['values']['foo']['bar'] will be.
+ *
+ * @see _form_validate()
+ */
+function _form_validate_skip($element, $form_state) {
+  // Validation of submitted form values can only be skipped, if the clicked
+  // button defines the #validate_values property. Otherwise, the entire form
+  // needs to be validated.
+  if (isset($form_state['clicked_button']['#validate_values'])) {
+    // Certain form buttons may want to skip form validation entirely, e.g. a
+    // "Previous" button in a multi-step form.
+    if (!$form_state['clicked_button']['#validate_values']) {
+      return TRUE;
+    }
+    // Compare all defined keys in #validate_values with the #parents of the
+    // passed $element. Since multiple form "sections" can be specified in
+    // #validate_values, we need to walk through all and only return FALSE if
+    // we find a direct match.
+    foreach ($form_state['clicked_button']['#validate_values'] as $section) {
+      if (array_slice($element['#parents'], 0, count($section)) === $section) {
+        return FALSE;
+      }
+    }
+    return TRUE;
   }
+  return FALSE;
 }
 
 /**
@@ -967,8 +1053,9 @@ function form_execute_handlers($type, &$
   if (isset($form_state[$type . '_handlers'])) {
     $handlers = $form_state[$type . '_handlers'];
   }
-  // Otherwise, check for a form-level handler.
-  elseif (isset($form['#' . $type])) {
+  // Otherwise, check for a form-level handler, but only if the complete form
+  // was validated.
+  elseif (isset($form['#' . $type]) && !isset($form_state['clicked_button']['#validate_values'])) {
     $handlers = $form['#' . $type];
   }
   else {
@@ -1006,7 +1093,7 @@ function form_execute_handlers($type, &$
  * @param $message
  *   The error message to present to the user.
  * @return
- *   Return value is for internal use only. To get a list of errors, use 
+ *   Return value is for internal use only. To get a list of errors, use
  *   form_get_errors() or form_get_error().
  */
 function form_set_error($name = NULL, $message = '') {
@@ -3261,7 +3348,7 @@ function batch_process($redirect = NULL,
   $batch =& batch_get();
 
   drupal_theme_initialize();
-  
+
   if (isset($batch)) {
     // Add process information
     $process_info = array(
@@ -3276,7 +3363,7 @@ function batch_process($redirect = NULL,
     );
     $batch += $process_info;
 
-    // The batch is now completely built. Allow other modules to make changes to the 
+    // The batch is now completely built. Allow other modules to make changes to the
     // batch so that it is easier to reuse batch processes in other enviroments.
     drupal_alter('batch', $batch);
 

=== modified file 'modules/field/field.form.inc'
--- modules/field/field.form.inc	2009-12-21 13:47:31 +0000
+++ modules/field/field.form.inc	2009-12-30 19:31:59 +0000
@@ -214,6 +214,7 @@ function field_multiple_value_form($fiel
           '#name' => $field_name . '_add_more',
           '#value' => t('Add another item'),
           '#attributes' => array('class' => array('field-add-more-submit')),
+          '#validate_values' => array(array($field_name, $langcode)),
           // Submit callback for disabled JavaScript.
           '#submit' => array('field_add_more_submit'),
           '#ajax' => array(

=== modified file 'modules/poll/poll.module'
--- modules/poll/poll.module	2009-12-26 16:50:08 +0000
+++ modules/poll/poll.module	2009-12-30 19:32:39 +0000
@@ -273,6 +273,7 @@ function poll_form($node, &$form_state) 
     '#value' => t('More choices'),
     '#description' => t("If the amount of boxes above isn't enough, click here to add more choices."),
     '#weight' => 1,
+    '#validate_values' => array(array('choice')),
     '#submit' => array('poll_more_choices_submit'), // If no javascript action.
     '#ajax' => array(
       'callback' => 'poll_choice_js',
@@ -901,4 +902,3 @@ function poll_user_cancel($edit, $accoun
       break;
   }
 }
-

=== modified file 'modules/simpletest/tests/form.test'
--- modules/simpletest/tests/form.test	2009-12-17 17:18:02 +0000
+++ modules/simpletest/tests/form.test	2009-12-30 19:31:59 +0000
@@ -231,6 +231,27 @@ class FormValidationTestCase extends Dru
     $this->assertNoFieldByName('name', t('Form element was hidden.'));
     $this->assertText('Name value: element_validate_access', t('Value for inaccessible form element exists.'));
   }
+
+  /**
+   * Tests partial form validation through #validate_values.
+   */
+  function testValidateValues() {
+    $edit = array('test' => 'pass');
+    $path = 'form-test/validate-values';
+
+    // Submit the form by pressing the button with #validate_values and ensure
+    // that the title field is not validated, but the #element_validate handler
+    // for the 'test' field is triggered.
+    $this->drupalPost($path, $edit, t('Partial validate'));
+    $this->assertNoText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertText('Test is validated');
+
+    // Now test full form validation and ensure that the #element_validate
+    // handler is still triggered.
+    $this->drupalPost($path, $edit, t('Full validate'));
+    $this->assertText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertText('Test is validated');
+  }
 }
 
 /**

=== modified file 'modules/simpletest/tests/form_test.module'
--- modules/simpletest/tests/form_test.module	2009-12-17 17:18:02 +0000
+++ modules/simpletest/tests/form_test.module	2009-12-30 19:31:59 +0000
@@ -17,6 +17,13 @@ function form_test_menu() {
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
   );
+  $items['form-test/validate-values'] = array(
+    'title' => 'Form validation values test_form',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('form_test_validate_values_form'),
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
 
   $items['form_test/tableselect/multiple-true'] = array(
     'title' => 'Tableselect checkboxes test',
@@ -204,6 +211,40 @@ function form_test_validate_form_validat
 }
 
 /**
+ * Builds a simple form with a button triggering partial validation.
+ */
+function form_test_validate_values_form($form, &$form_state) {
+  $form['title'] = array(
+    '#type' => 'textfield',
+    '#title' => 'Title',
+    '#required' => TRUE,
+  );
+  $form['test'] = array(
+    '#type' => 'textfield',
+    '#element_validate' => array('form_test_validate_values_form_validate_test'),
+  );
+  $form['actions']['partial'] = array(
+    '#type' => 'submit',
+    '#validate_values' => array(array('test')),
+    '#value' => t('Partial validate'),
+  );
+  $form['actions']['full'] = array(
+    '#type' => 'submit',
+    '#value' => t('Full validate'),
+  );
+  return $form;
+}
+
+/**
+ * Form element validation handler for the 'test' element.
+ */
+function form_test_validate_values_form_validate_test($form, &$form_state) {
+  if ($form_state['values']['test'] == 'pass') {
+    drupal_set_message('Test is validated');
+  }
+}
+
+/**
  * Create a header and options array. Helper function for callbacks.
  */
 function _form_test_tableselect_get_data() {
@@ -895,7 +936,7 @@ function form_test_state_persist($form, 
 
 /**
  * Submit handler.
- * 
+ *
  * @see form_test_state_persist()
  */
 function form_test_state_persist_submit($form, &$form_state) {
@@ -905,7 +946,7 @@ function form_test_state_persist_submit(
 
 /**
  * Implements hook_form_FORM_ID_alter().
- * 
+ *
  * @see form_test_state_persist()
  */
 function form_test_form_form_test_state_persist_alter(&$form, &$form_state) {