add a class to the body tag identifying the blog on a wordpress multi site install

wp_ms_body_class_site_id.php

<?php

add_action( 'body_class', 'my_body_class' ); 
function my_body_class( $class ) { 
    global $current_blog; 
    $class[] = 'site-' . $current_blog-> blog_id; 
    return $class; 
}  

?>