Get id of deleted category [Magento]

How do I get the id of a deleted category from an Observer?

config.xml markup

<adminhtml>
    <events>
        <catalog_category_delete_after>
            <observers>
                <my_module>
                    <type>singleton</type>
                    <class>my_module/observer</class>
                    <method>onCategoryDeleteAfter</method>
                </my_module>
            </observers>
        </catalog_category_delete_after>
    </events>
</adminhtml>

Observer code would like something like:

class My_Module_Model_Observer
{
    public function onCategoryDeleteAfter() {    
        #do something with deleted category id
    }
}

Use the passed $observer variable.

class My_Module_Model_Observer
{
    public function onCategoryDeleteAfter($observer) {
        $observer->getEvent()->getCategory();
    }
}