config.xml

<?xml version="1.0"?>
<config>
    <frontend>
        <events>
            <controller_action_predispatch_checkout_cart_add>
                <observers>
                    <extra_static>
                        <type>singleton</type>
                        <class>extra_static/observer</class>
                        <method>controllerActionPredispatchForceValidFormKey</method>
                    </extra_static>
                </observers>
            </controller_action_predispatch_checkout_cart_add>
            <controller_action_predispatch_wishlist_index_add>
                <observers>
                    <extra_static>
                        <type>singleton</type>
                        <class>extra_static/observer</class>
                        <method>controllerActionPredispatchForceValidFormKey</method>
                    </extra_static>
                </observers>
            </controller_action_predispatch_wishlist_index_add>
        </events>
    </frontend>
</config>

Observer.php

<?php

/**
 * Class Extra_Static_Model_Observer
 *
 * @category Model
 * @package  Extra_Static
 * @author   AOE Magento Team <[email protected]>
 * @license  none none
 * @link     www.aoe.com
 */
class Extra_Static_Model_Observer
{
    /**
     * forces the correct form_key
     *
     * @param Varien_Event_Observer $observer The varien event observer
     * @return void
     */
    public function controllerActionPredispatchForceValidFormKey(Varien_Event_Observer $observer)
    {
        $observer->getControllerAction()->getRequest()->setParam('form_key', Mage::getSingleton('core/session')->getFormKey());
    }
}