Ajax跨子域

<button onclick="crossDomain();">开始跨域</button>
<div id="ajax"></div>
<iframe src="http://work.2fool.cn/crossdomain/iframe.html" id="iframe" style="display:none;">
</iframe>
<script type="text/javascript">
document.domain = '2fool.cn';
function crossDomain(){
    var iframe=document.getElementById('iframe').contentWindow.$;        
    iframe.get("http://work.2fool.cn/crossdomain/helloworld.txt",function(data){
        document.getElementById("ajax").innerHTML=data;
    });
}
</script>

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript">
document.domain = '2fool.cn';
</script>