You could try something like this at the top of your iframe file:
PHP Code:
$mydomain = 'yourdomain.com';
if(strpos($_SERVER['HTTP_HOST'], $mydomain) === FALSE) {
header('Location: http://www.yourdomain.com');
}
The only problem is, because it's an iframe I'm not sure if $_SERVER will be populated with your server data or the data of the server belonging to the person linking it but you can test that pretty simply.
If it does cause a problem you could probably solve it by using css overflow to create a virtual iframe and including the file instead (I'm pretty sure an included file gets the server data of the calling page, not it's own server data), but if it gets to that point you may as well look at authentication as an easier solution.