gistfile1.php

/**
 * Make a CCK field read-only on form edition.
 *
 * @param $form
 *   The form, from hook_form_alter().
 * @param $field_name
 *   The name of the field to make read-only.
 */
function readonly_field(&$form, $field_name) {
  if (empty($form[$field_name])) {
    return;
  }

  // The original field structure.
  $field_structure = $form[$field_name];

  // Load the field definition.
  $field = content_fields($field_structure['#field_name'], $field_structure['#type_name']);

  $new_structure = array(
    '#tree' => TRUE,
    '#weight' => $field_structure['#weight'],
  );

  foreach (element_children($field_structure) as $sub_element) {
    $element = $field_structure[$sub_element];
    // Transform the field into an item.
    $new_structure[$sub_element . '_display'] = array(
      '#type' => 'item',
      '#title' => $element['#title'],
      '#value' => content_format($field, $element['#default_value'], $field['display_settings']['full']['format']),
    );
    $new_structure[$sub_element] = array(
      '#type' => 'value',
      '#value' => $element['#default_value'],
    );
  }

  $form[$field_name] = $new_structure;
}