<?php
$categories = get_the_category($post->ID);
if ($categories) {
$category_ids = array();
foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
$args=array( 'category__in' => $category_ids,
'post__not_in' => array($post->ID),
'showposts'=>5, // Posts a mostrar
'caller_get_posts'=>1
);

$original_query = $wp_query;
$wp_query = null;
$wp_query = new WP_Query( $args );

if ( have_posts() ) : ?>
 <h3>Posts Relacionados</h3><ul>
 <?php while ( have_posts() ) : the_post(); ?>
  <li><?php the_post_thumbnail( 'thumbnail' ); ?><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute();  ?>"><?php the_title(); ?></a></li> 
<?php   
 endwhile;
 echo '</ul>';
endif;

$wp_query = null;
$wp_query = $original_query;
wp_reset_postdata();
?>