From http://wordpress.org/support/topic/getting-post-thumbnail-url

post thumbnail

<?php query_posts('category_name=Portfolio&order=ASC&orderby=date&posts_per_page=36'); ?>

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>

<?php
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail' );
$url = $thumb['0'];
?>
<li><a style="background-image:url(<?=$url?>);" href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"></a></li>

<?php endwhile; ?>
<?php endif; ?>