config.xml

<?xml version="1.0"?>
<config>
    <modules>
	<MyCompany_MyModule>
	    <version>1.0.0</version>
	</MyCompany_MyModule>
    </modules>
    <global>
	<models>
	    <cron>
		<rewrite>
		    <observer>MyCompany_MyModule_Model_Observer</observer>
		</rewrite>
	    </cron>
	</models>
    </global>
</config>

Observer.php

<?php

class MyCompany_MyModule_Model_Observer extends Mage_Cron_Model_Observer
{

    /**
     * Process cron queue
     * Geterate tasks schedule
     * Cleanup tasks schedule
     *
     * @param Varien_Event_Observer $observer
     */
    public function dispatch($observer)
    {
        // DO YOUR STUFF HERE
        parent::dispatch($observer);
    }

}