jQuery: agregar jQuery Dinamicamente

<script type="text/javascript">
if (typeof jQuery == 'undefined') {
    var script = document.createElement('script');
    script.type = "text/javascript";
    script.src = "http://ajax.googleapis.com/ajax/libs/jquery/1.5.2/jquery.min.js";
    document.getElementsByTagName('head')[0].appendChild(script);
}
</script>