How to get Magento extension to run javascript when item added to cart -

i'm developing magento extension , having trouble trying run javascript when item added cart.

in config.xml have observer

<checkout_cart_product_add_after> 

in observer.php

public function itemaddedtocart(varien_event_observer $observer){} 

this firing (i can test echo-ing). how inject block of javascript (preferably footer)?

thanks in advance.

update: used chris' solution, instead of registry used session avoid potential multi-user issues:

in observer.php

public function itemaddedtocart(varien_event_observer $observer){}     $itemaddedtocart = 'true';     mage::getsingleton('core/session')->setitemaddedtocart($itemaddedtocart); 

in custom block (script.phtml):

<?php if($itemaddedtocart) : ?>     <script type="text/javascript">         alert(<?php echo '"' . $this->__($itemaddedtocart) . '"' ?>);     </script>     <?php          // clear itemaddedtocart session variable:         mage::getsingleton('core/session')->unsitemaddedtocart();      ?> <?php endif; ?> 

hope helps others.

you should able accomplish via xml in layout xml file of extension.

<catalog_product_view>     <reference name="footer">         <block type="core/template" name="insert_custom_name_here" template="path/to/your/phtml/file.phtml" />     </reference> </catalog_product_view> 

make sure delete cache because xml updates applied after refreshing cache.

if using default magento theme don't need call template because automatically calls children $this->getchildhtml('');. if using custom theme should add footer.phtml: $this->getchildhtml('insert_custom_name_here');.

or if you'd have javascript on cart page should replace <catalog_product_view> <checkout_cart_index>.