How to ajaxify simplenews module?
I don't know if you found your answer but I had the same problem and found a solution :

In a hook_form_alter

  if ($form_id == 'simplenews_block_form_' . variable_get('simplenews_id', 23)) {
    $form['submit']['#ajax'] = array(
      'callback' =&gt; 'simplenews_block_form_ajax_submit',
      'wrapper' =&gt; 'block-simplenews-' . variable_get('simplenews_id', 23),
      'method' =&gt; 'replace',
      'effect' =&gt; 'fade',
      'progress' =&gt; array('type' =&gt; 'none'),
    );
    $form['submit']['#executes_submit_callback'] = TRUE;
    unset($form['#submit']);
    unset($form['#validate']);
  }


And then in the ajax submit

/**
 * Ajax callback to reload the image field
 */
function simplenews_block_form_ajax_submit($form, $form_state) {
  $return = '&lt;div id="block-simplenews-23" class="block block-simplenews first odd"&gt;';
  $return .= '&lt;h2 class="block__title block-title"&gt;' . t('Newsletter') . '&lt;/h2&gt;';
  $return .= render($form);
  $return .= '&lt;/div&gt;';
  if (!valid_email_address($form_state['values']['mail'])) {
    $return .= '&lt;section class="errors-primal"&gt;' . t('The e-mail address is not valid.') . '&lt;/section&gt;';
    return $return;
  }
  else {
    if (module_exists('simplenews')) {
      module_load_include('inc', 'simplenews', 'views/simplenews.subscription');
      $tid = $form['#tid'];
      $account = simplenews_load_user_by_mail($form_state['values']['mail']);
      $confirm = simplenews_require_double_opt_in($tid, $account);
      $subscription = simplenews_subscribe_user($form_state['values']['mail'], $tid, $confirm, 'website');
      $return .= '&lt;section class="confirm-primal"&gt;' . t('You have been subscribed.') . '&lt;/section&gt;';
      return $return;
    }
  }
}


This will do a simple registration to the database in AJAX. You can adapt depending on what you exactly need.