WordPress: Listando posts agrupados por autor

<?php
$authors = $wpdb->get_results( $wpdb->prepare("SELECT ID, user_nicename from $wpdb->users WHERE 1=1 ORDER BY display_name") );

    if ( $authors ) {
        foreach ( $authors as $author ) {
          $user = get_userdata($author->ID);
          $args=array(
            'author' => $user->ID,
            'post_type' => 'post',
            'post_status' => 'publish',
            'posts_per_page' => -1,
            'caller_get_posts'=> 1
          );

          $my_query = null;
          $my_query = new WP_Query($args);

          if( $my_query->have_posts() ) {

            echo get_avatar( $user->ID, 46 ); //opcional

            echo '<h2>Posts de ' . $user->user_firstname . ' ' . $user->user_lastname . '</h2>';

            echo '<ul>';

            while ($my_query->have_posts()) : $my_query->the_post();

              echo '<li><a href="'. get_permalink() .'" rel="bookmark" title="Permanent Link to '. the_title_attribute('echo=0') .'">'. get_the_title(). '</a></li>';

            endwhile;

            echo '</ul>';

          }

          wp_reset_query();
         }
      }
?>