Insert content into iFrame

You really don’t need jQuery for that:

var doc = document.getElementById('iframe').contentWindow.document;
doc.open();
doc.write('Test');
doc.close();

jsFiddle Demo

If you absolutely have to use jQuery, you should use contents():

var $iframe = $('#iframe');
$iframe.ready(function() {
    $iframe.contents().find("body").append('Test');
});

jsFiddle Demo

Please don’t forget that if you’re using jQuery, you’ll need to hook into the DOMReady function as follows:

$(function() {  
    var $iframe = $('#iframe');
    $iframe.ready(function() {
        $iframe.contents().find("body").append('Test');
    });
});