Drupal: perform any action when a node is published

gistfile1.php

<?php

/**
 * Implementation of hook_nodeapi()
 * Perform any action when a node is published
 * Note: because no extra data added via $op = 'load', a node "reload" is necessary on presave
 */
function mymodule_nodeapi(&$node, $op) {
  if ($op == 'presave') {
    if ($node->type == 'my_node_type' && $node->status == 1) {
      // Get node state as of node_load()
      $n = node_load($node->nid);
		    if ($n->status == 0) {
		      //the node has just been published. do something
		    }
		  }
  }
}