function externalLinks() {
   if (!document.getElementsByTagName) return;
   var anchors = document.getElementsByTagName("a");
   for (var i=0; i<anchors.length; i++) {
      var anchor = anchors[i];
      if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") {
         anchor.target = "_blank";
      }
   }
}

function do_onload(){
   externalLinks();
}

if (window.addEventListener)
   window.addEventListener("load", do_onload, false)
else if (window.attachEvent)
   window.attachEvent("onload", do_onload)
else if (document.getElementById)
   window.onload=do_onload