config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Spl_Navigation>
            <version>0.0.1</version>
        </Spl_Navigation>
    </modules>
    <global>

    </global>
    <frontend>
        <events>
            <page_block_html_topmenu_gethtml_before>
                <observers>
                    <navigation_page_block_html_topmenu_gethtml_before>
                        <type>singleton</type>
                        <class>spl_navigation/observer</class>
                        <method>topmenuGethtmlBefore</method>
                    </navigation_page_block_html_topmenu_gethtml_before>
                </observers>
            </page_block_html_topmenu_gethtml_before>
        </events>
    </frontend>
</config>

Observer.php

<?php
/**
 * Created by JetBrains PhpStorm.
 * User: npreuss
 * Date: 4/8/13
 * Time: 12:19 PM
 * To change this template use File | Settings | File Templates.
 */
class Spl_Navigation_Model_Observer {
    public function topmenuGethtmlBefore (Varien_Event_Observer $observer) {
        $event = $observer->getEvent();
        Mage::log($event->getMenu(), null, 'fsi.log');
    }
}

Spl_Navigation.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Spl_Navigation>
            <active>true</active>
            <codePool>local</codePool>
        </Spl_Navigation>
    </modules>
</config>