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";
 }
}
window.onload = function() {
externalLinks();
}

/*THE ABOVE SHOULD BE FULLY FUNCTIONAL IN IE AND FF*/

/*
OTHER OPTIONS

///////////////    01    ///////////////

ONLY WORKS IN FIREFOX?

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";
 }
}
window.onload = externalLinks;

///////////////    02    ///////////////

<a href="file.cfm" onclick="javascript:window.open(this.href);return false;" title="click me">Click here</a>
It's XHTML valid, but DOES rely on the user having Javascript enabled.
*/