Redirect controller action (e.g. within observer predispatch)

redirect controller

public function yourMethod($observer)
{
    if ($condition) {
        $request = Mage::app()->getRequest();
        $request->initForward()
            ->setControllerName('controllername')
            ->setModuleName('modulename')
            ->setActionName('actionname')
            ->setDispatched(false);

        return false;
    }
}