Nav Menus

I have been unable to get theme locations working effectively using wordpress.

Here is my code:

register_nav_menus(
     array(
	 'main_navigation' => 'Main Navigation',
         'footer_navigation' => 'Footer Navigation'
     )
);

And calling them like so:

<?php $navargs = array( 'container' => '', 'menu_id' => 'main_nav', 'menu_location'  => 'Main Navigation' ); ?>
<?php wp_nav_menu($navargs); ?>

<?php
$footer_menu_args = array( 'container' => '', 'menu_id' => 'footer_nav', 'menu_location'  => 'Footer Navigation' );
wp_nav_menu($footer_menu_args);
?>

They show up fine in the admin section fine, but trying to add 2 different menus, they will display the same one.

wtf am I doing wrong.

Cheers
Ben