Twitter bootstrap modal не загружает внешние URL внутри модального.
Пример кода: jsFiddle
<a data-toggle="modal" class="btn" href="#" onclick="location.href='http://stackoverflow.com'; return false;" data-target="#myModal">click me</a>
<div class="modal hide fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
<h3 id="myModalLabel">Modal header</h3>
</div>
<div class="modal-body">
</div>
</div>