Why would post_type be ignored in this query?

Check if your post_type is exactly listings and not listing. Also check this approach:

<?php
function filter_where( $where="" ) {
    $where .= " AND post_date > '2009-03-01'";
    return $where;
}
add_filter( 'posts_where', 'filter_where' );

function parse_wp_query( $query ) {
    $query->set( 'post_type', array( 'listings' ) );
    return $query;
}
add_filter( 'parse_query', 'parse_wp_query' );

$posts = query_posts( 'posts_per_page=-1&post_type=listings' );

remove_filter( 'posts_where', 'filter_where' );
remove_filter( 'parse_query', 'parse_wp_query' );

echo '<ol>';
foreach ($posts as $post) {
    echo '<li>' . get_the_title() . '</li>';
}
echo '</ol>';
?>