With Simple PHP Proxy, your JavaScript can access content in remote webpages, without cross-domain security limitations, even if it's not available in JSONP format. Of course, you'll need to install this PHP script on your server, but that's a small price to have to pay for this much awesomeness.

<?php

// Script: Simple PHP Proxy: Get external HTML, JSON and more!
//
// *Version: 1.6, Last updated: 1/24/2009*
//
// Project Home - http://benalman.com/projects/php-simple-proxy/
// GitHub       - http://github.com/cowboy/php-simple-proxy/
// Source       - http://github.com/cowboy/php-simple-proxy/raw/master/ba-simple-proxy.php
//
// About: License
//
// Copyright (c) 2010 "Cowboy" Ben Alman,
// Dual licensed under the MIT and GPL licenses.
// http://benalman.com/about/license/
//
// About: Examples
//
// This working example, complete with fully commented code, illustrates one way
// in which this PHP script can be used.
//
// Simple - http://benalman.com/code/projects/php-simple-proxy/examples/simple/
//
// About: Release History
//
// 1.6 - (1/24/2009) Now defaults to JSON mode, which can now be changed to
//       native mode by specifying ?mode=native. Native and JSONP modes are
//       disabled by default because of possible XSS vulnerability issues, but
//       are configurable in the PHP script along with a url validation regex.
// 1.5 - (12/27/2009) Initial release
//
// Topic: GET Parameters
//
// Certain GET (query string) parameters may be passed into ba-simple-proxy.php
// to control its behavior, this is a list of these parameters.
//
//   url - The remote URL resource to fetch. Any GET parameters to be passed
//     through to the remote URL resource must be urlencoded in this parameter.
//   mode - If mode=native, the response will be sent using the same content
//     type and headers that the remote URL resource returned. If omitted, the
//     response will be JSON (or JSONP). <Native requests> and <JSONP requests>
//     are disabled by default, see <Configuration Options> for more information.
//   callback - If specified, the response JSON will be wrapped in this named
//     function call. This parameter and <JSONP requests> are disabled by
//     default, see <Configuration Options> for more information.
//   user_agent - This value will be sent to the remote URL request as the
//     `User-Agent:` HTTP request header. If omitted, the browser user agent
//     will be passed through.
//   send_cookies - If send_cookies=1, all cookies will be forwarded through to
//     the remote URL request.
//   send_session - If send_session=1 and send_cookies=1, the SID cookie will be
//     forwarded through to the remote URL request.
//   full_headers - If a JSON request and full_headers=1, the JSON response will
//     contain detailed header information.
//   full_status - If a JSON request and full_status=1, the JSON response will
//     contain detailed cURL status information, otherwise it will just contain
//     the `http_code` property.
//
// Topic: POST Parameters
//
// All POST parameters are automatically passed through to the remote URL
// request.
//
// Topic: JSON requests
//
// This request will return the contents of the specified url in JSON format.
//
// Request:
//
// > ba-simple-proxy.php?url=http://example.com/
//
// Response:
//
// > { "contents": "<html>...</html>", "headers": {...}, "status": {...} }
//
// JSON object properties:
//
//   contents - (String) The contents of the remote URL resource.
//   headers - (Object) A hash of HTTP headers returned by the remote URL
//     resource.
//   status - (Object) A hash of status codes returned by cURL.
//
// Topic: JSONP requests
//
// This request will return the contents of the specified url in JSONP format
// (but only if $enable_jsonp is enabled in the PHP script).
//
// Request:
//
// > ba-simple-proxy.php?url=http://example.com/&callback=foo
//
// Response:
//
// > foo({ "contents": "<html>...</html>", "headers": {...}, "status": {...} })
//
// JSON object properties:
//
//   contents - (String) The contents of the remote URL resource.
//   headers - (Object) A hash of HTTP headers returned by the remote URL
//     resource.
//   status - (Object) A hash of status codes returned by cURL.
//
// Topic: Native requests
//
// This request will return the contents of the specified url in the format it
// was received in, including the same content-type and other headers (but only
// if $enable_native is enabled in the PHP script).
//
// Request:
//
// > ba-simple-proxy.php?url=http://example.com/&mode=native
//
// Response:
//
// > <html>...</html>
//
// Topic: Notes
//
// * Assumes magic_quotes_gpc = Off in php.ini
//
// Topic: Configuration Options
//
// These variables can be manually edited in the PHP file if necessary.
//
//   $enable_jsonp - Only enable <JSONP requests> if you really need to. If you
//     install this script on the same server as the page you're calling it
//     from, plain JSON will work. Defaults to false.
//   $enable_native - You can enable <Native requests>, but you should only do
//     this if you also whitelist specific URLs using $valid_url_regex, to avoid
//     possible XSS vulnerabilities. Defaults to false.
//   $valid_url_regex - This regex is matched against the url parameter to
//     ensure that it is valid. This setting only needs to be used if either
//     $enable_jsonp or $enable_native are enabled. Defaults to '/.*/' which
//     validates all URLs.
//
// ############################################################################

// Change these configuration options if needed, see above descriptions for info.
$enable_jsonp = false;
$enable_native = false;
$valid_url_regex = '/.*/';

// ############################################################################

$url = $_GET['url'];

if (!$url) {

  // Passed url not specified.
  $contents = 'ERROR: url not specified';
    $status = array('http_code' => 'ERROR');
} elseif (!preg_match($valid_url_regex, $url)) {

  // Passed url doesn't match $valid_url_regex.
  $contents = 'ERROR: invalid url';
    $status = array('http_code' => 'ERROR');
} else {
    $ch = curl_init($url);

    if (strtolower($_SERVER['REQUEST_METHOD']) == 'post') {
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $_POST);
    }

    if ($_GET['send_cookies']) {
        $cookie = array();
        foreach ($_COOKIE as $key => $value) {
            $cookie[] = $key.'='.$value;
        }
        if ($_GET['send_session']) {
            $cookie[] = SID;
        }
        $cookie = implode('; ', $cookie);

        curl_setopt($ch, CURLOPT_COOKIE, $cookie);
    }

    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt($ch, CURLOPT_HEADER, true);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    curl_setopt($ch, CURLOPT_USERAGENT, $_GET['user_agent'] ? $_GET['user_agent'] : $_SERVER['HTTP_USER_AGENT']);

    list($header, $contents) = preg_split('/([\r\n][\r\n])\\1/', curl_exec($ch), 2);

    $status = curl_getinfo($ch);

    curl_close($ch);
}

// Split header text into an array.
$header_text = preg_split('/[\r\n]+/', $header);

if ($_GET['mode'] == 'native') {
    if (!$enable_native) {
        $contents = 'ERROR: invalid mode';
        $status = array('http_code' => 'ERROR');
    }

  // Propagate headers to response.
  foreach ($header_text as $header) {
      if (preg_match('/^(?:Content-Type|Content-Language|Set-Cookie):/i', $header)) {
          header($header);
      }
  }

    print $contents;
} else {

  // $data will be serialized into JSON data.
  $data = array();

  // Propagate all HTTP headers into the JSON data object.
  if ($_GET['full_headers']) {
      $data['headers'] = array();

      foreach ($header_text as $header) {
          preg_match('/^(.+?):\s+(.*)$/', $header, $matches);
          if ($matches) {
              $data['headers'][ $matches[1] ] = $matches[2];
          }
      }
  }

  // Propagate all cURL request / response info to the JSON data object.
  if ($_GET['full_status']) {
      $data['status'] = $status;
  } else {
      $data['status'] = array();
      $data['status']['http_code'] = $status['http_code'];
  }

  // Set the JSON data object contents, decoding it from JSON if possible.
  $decoded_json = json_decode($contents);
    $data['contents'] = $decoded_json ? $decoded_json : $contents;

  // Generate appropriate content-type header.
  $is_xhr = strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest';
    header('Content-type: application/'.($is_xhr ? 'json' : 'x-javascript'));

  // Get JSONP callback.
  $jsonp_callback = $enable_jsonp && isset($_GET['callback']) ? $_GET['callback'] : null;

  // Generate JSON/JSONP string
  $json = json_encode($data);

    print $jsonp_callback ? "$jsonp_callback($json)" : $json;
}