/**
 * Load jQuery from Google CDN, fallback to local
 */
// http://wp.tutsplus.com/tutorials/load-jquery-from-google-cdn-with-local-fallback-for-wordpress/
if( !is_admin()){ // Don't do this for admin area, since Google's jQuery isn't in noConflict mode and will interfere with WP's admin area.
	$url = 'http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js'; // the URL to check against
	$test_url = @fopen($url,'r'); // test parameters
	if($test_url !== false) { // test if the URL exists
	    function load_external_jQuery() { // load external file
	        wp_deregister_script( 'jquery' ); // deregisters the default WordPress jQuery
	        wp_register_script('jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js'); // register the external file
	        wp_enqueue_script('jquery'); // enqueue the external file
	    }
		add_action('wp_enqueue_scripts', 'load_external_jQuery'); // initiate the function
	} else {
	    function load_local_jQuery() {
	        wp_deregister_script('jquery'); // deregisters the default WordPress jQuery
	        wp_register_script('jquery', get_bloginfo('template_url').'/js/jquery-1.6.2.min.js', __FILE__, false, '1.6.2', true); // register the local file
	        wp_enqueue_script('jquery'); // enqueue the local file
	    }
	add_action('wp_enqueue_scripts', 'load_local_jQuery'); // initiate the function
	}
}

Add the following to your Wordpress theme's functions.php file. The jQuery paths/URLs might need to be changed to suite your circumstances.

Load jQuery from CDN, fallback to local (for Wordpress)

wordpress, jquery

PHP