Cut to the chase - Get the URL of the featured image, and optionally specify size

wp-featured-image-url.php

	function the_featured_image_url($post = 0, $size = 'full') {
		echo apply_filters('the_featured_image_url', get_featured_image_url($post, $size));
	}
	function get_featured_image_url($post = 0, $size = 'full') {
		$post = get_post($post);
		$thumbnail = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), $size);
		return apply_filters('get_featured_image_url', $thumbnail[0]);
	}