lesson learned from HTML5 boilerplate:
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="/front/try/js/vendor/jquery-1.8.0.min.js"><\/script>')</script>
The window.jQuery
will check if the cdn is not available, it will use your local jQuery as a plan B.