<?php
class My_Controller_PreDispatchPlugin extends Zend_Controller_Plugin_Abstract
{
    public function preDispatch(Zend_Controller_Request_Abstract $request)
    {
        $dispatcher = Zend_Controller_Front::getInstance()->getDispatcher();
        $controller = $dispatcher->getController($request);
        if (!$controller) {
            $controller = $dispatcher->getDefaultControllerName($request);
        }
        $action     = $dispatcher->getAction($request);

        if (!method_exists($controller, $action)) {
            $defaultAction = $dispatcher->getDefaultAction();
            $controllerName = $request->getControllerName();
            $response = Zend_Controller_Front::getInstance()->getResponse();
            $response->setRedirect('/' . $controllerName . '/' . $defaultAction);
            $response->sendHeaders();
            exit;
        }
    }
}

The dispatcher doesn't have a function getController() it does however have a getControllerClass which will return the className or false which is nice but still won't give you an initialized class to use for the action checking.

Also the headering to /:controller/:action is nice but it should be reverse routed first and have the $request->getBaseUrl() prepended.

I know it's example code but a proper working example would be nice.