WordPress' missing the_post_thumbnail_url() function.

the_post_thumbnail_url.php

<?php

/**
  * Outputs the URL of the "Featured Image", also known as "Post Thumbnail".
  * @param $post_id (optional) The ID of the Post whose Thumbnail you wish to output. When used within "The Loop", this parameter can be ommitted.
  * @return void
  */
function the_post_thumbnail_url($post_id = false) {
	global $post;
	
	if (!$post_id) {
		$post_id = $post->ID;
	}

	$thumbnail_id = get_post_thumbnail_id($post_id);
	echo wp_get_attachment_url($thumbnail_id);
}

?>