WordPress Must Use Plugin which rewrites asset URI's to CDN77.

cdn77-uri-rewrite.php

<?php
/**
 * WordPress Must Use Plugin which rewrites asset URI's to CDN77.
 *
 * This is largely inspired my Mark Jaquith's WP_Stack CDN plugin
 * https://github.com/markjaquith/WP-Stack/blob/master/WordPress-Dropins/wp-stack-cdn.php
 *
 * Options that should be set in wp-config.php
 *  CDN77_ENABLE - Set to TRUE if the CDN should be enabled
 *  CDN77_DOMAIN - The domain name for the CDN77 subdomain in use
 *                  (can be | separated for rotating domains)
 */

// Hook on init to setup the output buffering for this plugin
add_action( 'init', 'cdn77_rewrite_init', 5 );
if ( ! function_exists( 'cdn77_rewrite_init' ) ):

function cdn77_rewrite_init() {
    // Only bother if the CDN is enabled
    if ( defined( 'CDN77_ENABLED' ) && CDN77_ENABLED ) {
        add_action( 'template_redirect', 'cdn77_rewrite_buffer_start' );
    }
}

endif;

// Define action for starting and flushing output buffers
if ( ! function_exists( 'cdn77_rewrite_buffer_start' ) ):

function cdn77_rewrite_buffer_flush( $content ) {
    // setup the domains that will be replaced
    $extensions = array( 'jpe?g', 'gif', 'png', 'css', 'bmp', 'js', 'ico' );
    $domain = parse_url( get_bloginfo( 'url' ), PHP_URL_HOST );
    $cdn = ( defined( 'CDN77_DOMAIN' ) ) ? explode( '|', CDN77_DOMAIN ) : NULL; // just in case
    if ( is_null( $cdn ) ) {
        return $content;
    }
    // how many replacement domains have been specfied
    $nDomains = count($cdn); $cdnIdx = 0; $rCount = 1;
    if ( 0 === $nDomains ) {
        return preg_replace( "#=([\"'])(https?://{$domain})?/([^/](?:(?!\\1).)+)\.(" . implode( '|', $extensions ) . ")(\?((?:(?!\\1).)+))?\\1#",
            '=$1//' . $cdn[$cdnIdx] . '/$3.$4$5$1', $content );
    } else {
        // loop over the content until no more replacements are required
        while( $rCount > 0 ) {
            $content = preg_replace( "#=([\"'])(https?://{$domain})?/([^/](?:(?!\\1).)+)\.(" . implode( '|', $extensions ) . ")(\?((?:(?!\\1).)+))?\\1#",
                '=$1//' . $cdn[$cdnIdx] . '/$3.$4$5$1', $content, 1, $rCount );
            $cdnIdx = ( $cdnIdx + 1) % $nDomains;
        }
        // and finally output the modified content
        return $content;
    }
}

function cdn77_rewrite_buffer_start() {
    ob_start( 'cdn77_rewrite_buffer_flush' );
}

endif;