Pass user_register form field values to Pardot for extra processing
First you'd want to add an additional submit handler to your form...

function pardotpost_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'your_form_id_goes_here') {
    $form['#submit'][] = 'send_to_pardot';
  }  
}


Then in your new submit handler you post the data to their form handler

function send_to_pardot(&amp;$form, &amp;$form_state) {
  $url = 'http://pardot.com/your/form/handler/url/goes/here';
  $data = $form_state['values']['submitted_tree'];
  $options = array(
    'method' =&gt; 'POST',
    'data' =&gt; http_build_query($data, '', '&amp;'),
    'timeout' =&gt; 15,
    'headers' =&gt; array('Content-Type' =&gt; 'application/x-www-form-urlencoded'),
  );
  $result = drupal_http_request($url, $options);
}


Lastly you check $result for any errors.
Cheers