update_post_thumbnail_cache( WP_Query $wp_query = null )

function update_post_thumbnail_cache( $wp_query = null ) {
	if ( ! $wp_query )
		$wp_query = $GLOBALS['wp_query'];

	if ( $wp_query->thumbnails_cached )
		return;

	$thumb_ids = array();
	foreach ( $wp_query->posts as $post ) {
		if ( $id = get_post_thumbnail_id( $post->ID ) )
			$thumb_ids[] = $id;
	}

	if ( ! empty ( $thumb_ids ) ) {
		_prime_post_caches( $thumb_ids, false, true );
	}

	$wp_query->thumbnails_cached = true;
}