Drupal 7: Find when a node is first published. This uses workbench moderation.

template.php

<?php
/**
 * Implements hook_preprocess_node().
 */
function my_module_preprocess_node(&$variables) {
  $node = $variables['node'];
  // First published.
  // This query uses workbench moderation to determine the first moderation
  // transition that resulted in a published node and takes the timestamp
  // from that record.
  $result = db_query('
    SELECT m.stamp
    FROM {node_revision} r
    LEFT JOIN {node} n ON n.vid = r.vid
    INNER JOIN {workbench_moderation_node_history} m ON m.vid = r.vid
    WHERE r.nid = :nid
    AND m.state = :published_state
    ORDER BY m.stamp ASC
    LIMIT 1', array(
      ':nid' => $node->nid,
      ':published_state' => workbench_mod  eration_state_published(),
    ))->fetchObject();
  if (empty($result->stamp)) {
    $date = "Unpublished";
  }
  else {
    $date = format_date($result->stamp, 'long');                                                                                           
  }
  $variables['first_published'] = t('First published on !datetime', array('!datetime' => $date));
}