functions.php

/**
 * Change the URL for static files to use the CDN
 */
if (FIT_USE_CDN)
{
    add_filter('theme_root_uri', 'fit_cdn_rewrite', 99, 1);
    add_filter('wp_get_attachment_image_src', 'fit_cdn_image_rewrite', 99, 1);
    add_filter('the_content', 'fit_cdn_content_rewrite', 99, 1);
}

function fit_cdn_rewrite($url) {
    $url = str_replace(get_site_url(), FIT_CDN_URL, $url);
    return $url;
}

function fit_cdn_image_rewrite($image) {
    $image[0] = str_replace(get_site_url(), FIT_CDN_URL, $image[0]);
    return $image;
}

function fit_cdn_content_rewrite($content) {
    $content = str_replace(get_site_url() . '/wp-content/', FIT_CDN_URL . '/wp-content/', $content);
    return $content;
}

wp-config.php

define('FIT_USE_CDN', true);
define('FIT_CDN_URL', 'http://cdn.domain.com');