How to uses databases directive in aliases.drushrc

Hi there--
I'm trying to create an alias pointing to a different DB server than is in my settings.php.

I've got an aliases.drushrc.php file in my drush directory, it doesn't throw any errors when I try to use the alias, but it is not using the DB credentials I've specified in the file. I don't get any errors when I run the command and it says it's loading the alias, but according to the sql-dump file I'm generating, it's pulling from the server in settings.php

drush status
 Drupal version                :  7.8                                              
 Site URI                      :  http://default                                   
 Database driver               :  mysql                                            
 Database hostname             :  www-db.REDACTED                          
 Database username             :  REDACTED                                  
 Database name                 :  REDACTED_DB                                     
 Database                      :  Connected                                        
 Drupal bootstrap              :  Successful                                       
 Drupal user                   :  Anonymous                                        
 Default theme                 :  REDACTED                                     
 Administration theme          :  seven                                            
 PHP configuration             :  /etc/php5/cli/php.ini                            
 Drush version                 :  4.5                                              
 Drush configuration           :                                                   
 Drush alias files             :  /usr/share/drush/includes/../aliases.drushrc.php 
 Drupal root                   :  /srv/www/svn/trunk/public                        
 Site path                     :  sites/default                                    
 File directory path           :  sites/default/files                              
 Private file directory path   :

drush call:

drush @backup sql-dump > test.sql -vvvvv
Load alias @backup                                                                                                           [notice]
Initialized Drupal 7.8 root directory at /srv/www/svn/trunk/public                                                           [notice]
Initialized Drupal site default at sites/default                                                                             [notice]
Command dispatch complete                                                                                                    [notice]

head test.sql 
Calling system(mysqldump --single-transaction --opt -Q  DB_REDACTED --host=www-db.REDACTED --port=3306 --user=REDACTED --password=REDACTED );

aliases.drushrc.php definition of the alias:

<?php
$aliases['backup'] = array(
  'databases' => array(
     'default' => array(
        'default' => array(
          'driver' => 'mysql',
          'database' => 'REDACTED',
          'username' => 'REDACTED',
          'password' => 'REDACTED',
          'host' => 'DESIRED_HOST',
          'port' => 3306,
          'prefix' => '',
          'collation' => 'utf8_general_ci',
        )
      )
    )
);

Any demos or examples of how the 'databases' instruction in aliases is supposed to work?

Thx