Example of how to take a textarea form element and explode it into an array before saving it to a variable via `system_settings_form()`.

D6MODULE_textarea_to_array.php

<?php

/**
 * @file
 * Example of how to take a textarea form element and explode it into an array
 * before saving it to a variable via `system_settings_form()`.
 */

/**
 * Admin settings form.
 */
function D6MODULE_admin_settings_form() {
  $form = array();

  $form['D6MODULE_textarea_to_array'] = array(
    '#type' => 'textarea',
    '#value_callback' => 'D6MODULE_admin_textarea_to_array_value_callback',
    '#element_validate' => array('D6MODULE_admin_textarea_to_array_validate'),
  );

  return system_settings_form($form);
}

/**
 * Value callback for the `D6MODULE_textarea_to_array` field.
 */
function D6MODULE_admin_textarea_to_array_value_callback($element, $edit = FALSE) {
  if (func_num_args() == 1) {
    // No value yet, so use #default_value if available.
    if (isset($element['#default_value']) && is_array($element['#default_value'])) {
      // Convert to textarea value (key|value).
      $joined_values = array();
      foreach ($element['#default_value'] as $key => $value) {
        $joined_values[] = "$key|$value";
      }
      return implode("\n", $joined_values);
    }
    return '';
  }
  return $edit;
}

/**
 * Element validate callback for the `D6MODULE_textarea_to_array` field.
 */
function D6MODULE_admin_textarea_to_array_validate($element, &$form_state) {
  if (!empty($element['#value'])) {
    // Convert from textarea value to $values array.
    $values = array();
    foreach (explode("\n", trim($element['#value'])) as $value) {
      // Format of the textarea is key|value.
      list($key, $value) = explode('|', $value);
      $values[trim($key)] = trim($value);
    }

    // Check that all $values are valid (optional).
    if (!empty($values)) {
      // This is an example validation routine that checks whether the contents
      // of the array values reference valid CCK field names on a content type.
      $type_name = 'page';
      $type = content_types($type_name);
      foreach ($values as $value) {
        if (!isset($type['fields'][$value])) {
          form_error($element, t('All CCK fields must be valid CCK fields on the %type node type.', array('%type' => $type_name)));
          // Exit here on the first error found.
          return;
        }
      }
      // If no error, set the value on the element to the validated $values array.
      form_set_value($element, $values, $form_state);
    }
  }
}