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

Event.observe(window, 'load', externaliseLinks);