I just recently upgrade one of my magento ecommerce website from 1.7.x to 1.8.1 and one of my customer feedback that he encounter a javascript undefined error upon checkout.
After some comparison with the base folder of magento 1.8.1, i figured out that you need to amend the following page.
app/design/frontend/default/THEME_NAME/template/checkout/onepage/review/info.phtml
Search for the following line or similar
echo $this->getUrl('checkout/onepage/saveOrder');
and change it to
echo $this->getUrl('checkout/onepage/saveOrder', array('form_key' => Mage::getSingleton('core/session')->getFormKey()));