You can replace target=_blank with jquery, or prototype or with raw javascript if you like. Here is the code i came up with to do just this.

jQuery

$(function() {
	$('a[rel*=external]').click(function() {
		var w = window.open(this.href);
		if(!w) alert("Boo! A popup blocker stopped our window from opening");
		return false;
	});
});