This turned out to be my own monologue

Luckily I found a solution myself again by reading Zend/Layout.php file.
Here's how my preDispatch function looks now:
[PHP]
public function preDispatch(Zend_Controller_Request_Abstract $request)
{
$module = $request->getModuleName();
$controller = $request->getControllerName();
$action = $request->getActionName();

$layout = Zend_Layout::getMvcInstance();
$layout->getView()->headTitle()->prepend(SITE_TITLE);
$layout->getView()->headTitle()->setSeparator(' - ');

switch ($module) {
case 'default':
break;

case 'ajax':
$layout->disableLayout();
break;

default:
break;
}
}
[/PHP]

Hope this helps someone...