A quick tip to get your forms to play nice with admin routing:

/views/users/edit.ctp

<?php echo $form->create("User", array("url" => $html->url(array("admin" => true), true))) ?>

Why? For some reason, $form->create() ignores the usual routing prefix attribute ("admin"=>true) so you have to pass the url through $html->url() which does take prefix routing into account.

The last true param tells $html->url() to return an absolute url (from your site route).