si', e' lo script ad essere molto approssimativo

l' ho modificato per fargli fare quello che chiedi,
ma serve usare una versione piu' recente di jquery
o se vuoi usare quella vecchia devi aggiungere il plugin dimensions per ottenere scrollLeft e scrollTop
codice:
/* Greybox Redux
 * Required: http://jquery.com/
 * Written by: John Resig
 * Based on code by: 4mir Salihefendic (http://amix.dk)
 * License: LGPL (read more in LGPL.txt)
 */

var GB_DONE = false;
var GB_HEIGHT = 400;
var GB_WIDTH = 400;

function GB_show(caption, url, height, width) {
  GB_HEIGHT = height || 400;
  GB_WIDTH = width || 400;
  if(!GB_DONE) {

  	var wH=$(window).height();
	var dH=$(document).height();
	var maxH=(wH>dH)?wH:dH;

    $(document.body)
      .append("<div id='GB_overlay' style='height:"+maxH+"px'></div><div id='GB_window'><div id='GB_caption'></div>"
        + "[img]js/greybox/close.gif[/img]</div>");
    $("#GB_window img").click(GB_hide);
    $("#GB_overlay").click(GB_hide);
    $(window).resize(GB_position);
    GB_DONE = true;
  }

  $("#GB_frame").remove();
  $("#GB_window").append("<iframe id='GB_frame' src='"+url+"'></iframe>");

  $("#GB_caption").html(caption);
  $("#GB_overlay").show();
  GB_position();

  if(GB_ANIMATION)
    $("#GB_window").slideDown("slow");
  else
    $("#GB_window").show();
}

function GB_hide() {
  $("#GB_window,#GB_overlay").hide();
}

function GB_position() {
  var l = $('html').scrollLeft()+(($(window).width()-GB_WIDTH)/2);
  var t = $('html').scrollTop()+(($(window).height()-GB_HEIGHT)/2);
  l=(l<0)?0:l;
  t=(t<0)?0:t;
  $("#GB_window").css({width:GB_WIDTH+"px",height:GB_HEIGHT+"px",
    left: l+"px", top: t+"px" });
  $("#GB_frame").css("height",GB_HEIGHT - 32 +"px");
}
ciao