<!-- Grab Google CDN jQuery. fall back to local if necessary -->
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script>!window.jQuery && document.write('<script src="js/jquery-1.4.2.min.js"><\/script>')</script>

http://net.tutsplus.com/tutorials/javascript-ajax/14-helpful-jquery-tricks-notes-and-best-practices/

if window.jQuery is undefined, there must have been a problem downloading the script from the CDN. In that case, proceed to the right side of the && operator, and insert a script linking to a local version of jQuery.

Conditionally Loading jQuery

html, jquery

HTML