Efficient way to fetch all archived WPMS blogs

You can go directly to the database via $wpdb.

<?php
$wpdb->get_results( "SELECT * FROM $wpdb->blogs WHERE archived = 1" );

Or wrapped up in a function:

<?php
function wpse34731_get_archived_blogs()
{
    global $wpdb;
    if( empty( $wpdb ) ) return;

    $blogs = $wpdb->get_results( "SELECT * FROM $wpdb->blogs WHERE archived = 1" );
    return $blogs;
}