If you find that your custom theme no longer allows the Update Cart button to work after an upgrade to 1.8x then you need to make some alterations to you cart.phtml file.
<?php echo $this->getBlockHtml(‘formkey’); ?>
<form action=”<?php echo $this->getUrl(‘checkout/cart/updatePost’) ?>” method=”post”>
You may also need to do the same for your checkout review form / add to cart form / checkout register form
In 1.8 most of the Magento form submissions require form_key and Magneto checks for the form post destination controller, if the form_key is not present then the function does not execute any more.