Within the file /includes/modules/pages/checkout_payment/jscript_main.php, there's this section:
Code:
    $(document).ready(function(){
      $('form[name="checkout_payment"]').submit(function() {
          $('.paymentSubmit').attr('disabled', true);
        <?php if ($flagOnSubmit) { ?>
          formPassed = check_form();
          if (formPassed == false) {
              $('.paymentSubmit').attr('disabled', false);
          }
          return formPassed;
        <?php } ?>
      });
    });
... but if you look at /includes/templates/template_default/templates/tpl_checkout_payment_default.php, you'll see:
Code:
<div class="buttonRow forward" id="paymentSubmit"><?php echo zen_image_submit(BUTTON_IMAGE_CONTINUE_CHECKOUT, BUTTON_CONTINUE_ALT, 'onclick="submitFunction('.zen_user_has_gv_account($_SESSION['customer_id']).','.$order->info['total'].')"'); ?></div>
Suggest changing the jQuery code to reference the "proper" selector:
Code:
   $(document).ready(function(){
      $('form[name="checkout_payment"]').submit(function() {
          $('#paymentSubmit').attr('disabled', true);
        <?php if ($flagOnSubmit) { ?>
          formPassed = check_form();
          if (formPassed == false) {
              $('#paymentSubmit').attr('disabled', false);
          }
          return formPassed;
        <?php } ?>
      });
    });