After Magento CE 1.4.0.1, Mage_Checkout_Block_Onepage_Success::getOrderId() has been deprecated.
Need to get the order id?
Mage::getModel('checkout/session')->getLastRealOrderId()
Don’t bother looking for getLastRealOrderId() as it uses Magento’s magical (or not so magical) getter/setter. More about that here: http://codegento.com/2011/02/where-are-my-getters-and-setters/