<script type="text/javascript" src="http://code.jquery.com/jquery-latest.pack.js"></script>

var $ = parent.$;

$("#element", document).doStuff();

var $ = parent.$;

if (typeof(jQuery) == "undefined") {
    var iframeBody = document.getElementsByTagName("body")[0];
    var jQuery = function (selector) { return parent.jQuery(selector, iframeBody); };
    var $ = jQuery;
}