function externalLinks() 
{ 
    if (!document.getElementsByTagName) return; //Browserweiche
    var anchors = $A(document.getElementsByTagName("a"));
	anchors.each(function(anchor) {   
        if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") 
	    {
	        anchor.target = "_blank"; 
	    }
	});
}

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