Fall Back Google CDN in JavaScript

Scripts don’t get printed at the wp_enqueue_scripts hook, but rather at the wp_print_scripts hook. That said: don’t echo/print scripts, period. Instead, enqueue them properly.

Caveat: This method is Plugin territory, and should not be included as Theme code in a publicly distributed Theme.

You deregister jQuery, but the step you’re missing is registering and enqueueing your own version:

<?php
function sbi_cdn_jquery() {
    // Deregister core-bundled jQuery
    // This is dangerous! Understand the
    // potential consequences before doing this!
    wp_deregister_script( 'jquery' );

    // Register custom jQuery version
    wp_register_script( 'jquery', '//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js' );

    // Enqueue cusotm jQuery
    wp_enqueue_script( 'jquery' );
}
add_action( 'wp_enqueue_scripts', 'sbi_cdn_jquery' );
?>

Refer to the Codex entry for wp_register_script() and wp_enqueue_script() regarding arguments for these functions.