Jake Spurlock

function get_the_post_thumbnail_src( $post_id = NULL, $size = 'post-thumbnail', $attr = '' ) {

    global $id;
    $post_id = ( NULL === $post_id ) ? $id : $post_id;
    $post_thumbnail_id = get_post_thumbnail_id( $post_id );
    $size = apply_filters( 'post_thumbnail_size', $size );
    if ( $post_thumbnail_id ) {
        do_action( 'begin_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size ); // for "Just In Time" filtering of all of wp_get_attachment_image()'s filters
        $src = wp_get_attachment_image_src( $post_thumbnail_id, $size, false, $attr );
        do_action( 'end_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size );
    } else {
        $src = '';
    }
    return $src[0];
}

?>