To properly use the rewrite router you have to instantiate it, add some user defined routes and inject it into the controller. The following code illustrates the procedure:
<?php
// Create a router
$router = $ctrl->getRouter(); // returns a rewrite router by default
$router->addRoute(
'user',
new Zend_Controller_Router_Route('user/:username',
array('controller' => 'user',
'action' => 'info'))
);