A workaround for Cross-domain XHR's not working in Chrome Extensions' content scripts. See http://groups.google.com/group/chromium-extensions/browse_thread/thread/43ec4d383cf8d01d

background-page.html

<!DOCTYPE html>
<html>
<head>
<script src="xhrproxy.js"></script>
<script>

setupXHRProxy();

</script>
</head>
</html>

content-script.js

  ...

proxyXHR({
  method: 'GET',
  url: 'http://bar.com/external.js',
  onComplete: function(status, data) {
    if (status == 200) {
      alert(data);
    } else {
      alert("HTTP Error " + status + " while retrieving data.");
    }
  }
});

  ...

manifest.json

  ...

"background_page": "background.html",
"content_scripts": [
  {
    "matches": ["http://foo.com/*"],
    "js": ["xhrproxy.js", "content-script.js"]
  }
],
"permissions": [
  "http://bar.com/"
]

  ...

xhrproxy.js

var XHR_PROXY_PORT_NAME_ = 'XHRProxy_';

/**
 * Should be called by the background page.
 */
function setupXHRProxy() {
  chrome.extension.onConnect.addListener(function(port) {
    if (port.name != XHR_PROXY_PORT_NAME_)
      return;
    
    port.onMessage.addListener(function(xhrOptions) {
      var xhr = new XMLHttpRequest();
      xhr.open(xhrOptions.method || "GET", xhrOptions.url, true);
      xhr.onreadystatechange = function() {
        if (this.readyState == 4) {
          port.postMessage({
            status: this.status,
            data: this.responseText,
            xhr: this
          });
        }
      }
      xhr.send();
    });
  });
}

/**
 * Should be called by the content script.
 */
function proxyXHR(xhrOptions) {
  xhrOptions = xhrOptions || {};
  xhrOptions.onComplete = xhrOptions.onComplete || function(){};
  
  var port = chrome.extension.connect({name: XHR_PROXY_PORT_NAME_});
  port.onMessage.addListener(function(msg) {
    xhrOptions.onComplete(msg.status, msg.data, msg.xhr);
  });
  port.postMessage(xhrOptions);
}

Updated the script to work with manifest version 2
background.js

chrome.extension.onConnect.addListener(function(port) {
  if (port.name != 'XHRProxy_')
    return;
  port.onMessage.addListener(function(xhrOptions) {
    var xhr = new XMLHttpRequest();
    xhr.open(xhrOptions.method || "GET", xhrOptions.url, true);
    xhr.onreadystatechange = function() {
      if (this.readyState == 4) {
        port.postMessage({
          status : this.status,
          data   : this.responseText,
          xhr    : this
        });
      }
    }
    xhr.send();
  });
});

xhrproxy.js
The big change here is to the interface. It makes it so you can do proxyXHR.get('domain').onSuccess(callback) for a bit of a cleaner interface.

var proxyXHR = {};

proxyXHR.get = function (url) {
  var port     = chrome.extension.connect({ name: 'XHRProxy_' });
  var settings = {
    method : 'GET',
    url    : url
  };
  var onSuccess;
  var onFailure;
  var self = {
    onSuccess: function (callback) {
      onSuccess = callback;
      return self;
    },
    onFailure: function (callback) {
      onFailure = callback;
      return self;
    }
  };
  port.onMessage.addListener(function (msg) {
    if (msg.status === 200 && typeof onSuccess === 'function') {
      onSuccess(msg.data, msg.xhr);
    } else if (typeof onFailure === 'function') {
      onFailure(msg.data, msg.xhr);
    }
  });
  port.postMessage(settings);
  return self;
};

content-script.js

proxyXHR.get('http://bar.com/external.js').onSuccess(function (data) {
  alert(data);
}).onFailure(function (status) {
  alert("HTTP Error " + status + " while retrieving data.");
});

"background": {
  "scripts": ["background.js"]
},
"content_scripts": [{
  "matches" : ["http://*/*","https://*/*"],
  "js"      : ["xhrproxy.js", "content-script.js"],
}],

@SeanJM The msg.status value is always 0 and the msg.data value is always empty in your revision. Any ideas?

Hey eComEvo, If it didn't work I would not have posted it, I am currently using that code in a Chrome plugin with no problems.

How did you structure it?

I'm not having any success with this either in the latest version of Chrome. I can't even get onMessage to fire?