Google Library API - google.load无法从事件中加载？

<script type="text/javascript" src="https://www.google.com/jsapi?key=INSERT-YOUR-KEY"></script>
<script type="text/javascript">
    // --- LOADING LIKE THIS WORKS FINE ---
    // jQuery is completely loaded
    function jqueryLoaded() {
        $("body").css("background-color","orange");
    }
    google.load("jquery", "1.4.3");
    google.setOnLoadCallback(jqueryLoaded);     

    // --- LOADING FROM AN EVENT DOES NOT WORK?? ---
    // jQuery UI is completely loaded
    function jqueryUILoaded() {
        $("body").css("background-color","green");
    }   
    function loadJqueryUI() {
        alert("load jQuery UI now..");
        google.load("jqueryui", "1.8.6");
        google.setOnLoadCallback(jqueryUILoaded);
    }

    // with a setTimeout it doesn't work either.. 
    // setTimeout("loadJqueryUI()", 2000);
</script>  

<input type="button" value="load jQuery UI" onclick="loadJqueryUI()"/>