Most users will have the javascript in their cache anyway, so they won't even be hitting the CDN. Also, why not check to see if the object exists before doing anything else?

    <script id="jquery" src="http://code.google.com/jquery.js"></script>
    <script>
    if (jQuery == undefined) {
        document.querySelector("#jquery").src = 'Backup CDN';
    }
    </script>

That should work, in theory, if not then just remove script#jquery, create a new one and append to the document before executing any script(s).

Stolen from the html5 boilerplate

    <!-- Grab Google CDN's jQuery. fall back to local if necessary -->
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.js"></script>
    <script>!window.jQuery && document.write(unescape('%3Cscript src="js/libs/jquery-1.4.2.js"%3E%3C/script%3E'))</script>