Ok, the page runs now.
I tested in Chrome, FireFox, IE9 and IE10 with both the trunk and v2.2 Ext.NET. The Checkbox appears for me.
Is that all CSS and JavaScript stuff required to reproduce the problem? I mean
CSS
<link href="bootstrap2/css/bootstrap.css" rel="stylesheet">
<link href="bootstrap2/css/datepicker.css" rel="stylesheet">
<link href="bootstrap2/css/bootstrap-responsive.css" rel="stylesheet">
By the way, the things above have no closing tags.
JavaScript
<script type="text/javascript" src="bootstrap2/js/jquery.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-transition.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-alert.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-modal.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-dropdown.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-scrollspy.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-tab.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-tooltip.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-popover.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-button.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-collapse.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-carousel.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-typeahead.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-inputmask.js"></script>
<script type="text/javascript" src="bootstrap2/js/bootstrap-datepicker.js"></script>
Please try to remove it on your side and retest.