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") &&
       anchor.getAttribute("rel").indexOf("external") >= 0)
     anchor.target = "_blank";
 }
}

document.observe('dom:loaded', function() {
  externalLinks();
});