Thanks for the example.
It seems with ZF 1.9.1 I can do just this:

$hostnameRoute = new Zend_Controller_Router_Route_Hostname(
‘:username.domain.com’,
array(
‘module’ => ‘default’,
‘controller’ => ‘index’,
‘action’ => ‘index’,
),
array(‘username’ => ‘(?!www$).*’) // skip www as a username
);

$router->addRoute(‘user_route’, $hostnameRoute);