Message Passing attempt

content_script.js

chrome.extension.onRequest.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello")
      sendResponse({farewell: "goodbye"});
    else
      sendResponse({}); // snub them.
  });

manifest.json

{
  "name": "Test",
  "version": "1.0",
  // no more "background_page": "background.html",
  "browser_action": {
    "popup": "popup.html"
  },
  "permissions": [
    "tabs", "http://*/*"
  ],
  "content_scripts": [
    {
      "matches": ["http://*/*"],
      "js": ["content_script.js"]
    }
  ]  
}

popup.html

<script>
// Note that it's now popup.html instead of background.html. If you put
// it at the top of background.html, then it will run only ONCE, the first
// time the extension is loaded.
//
// You can put this call in the background if you really wanted, but it
// should be triggered by some event such as chrome.browserAction.onClicked.
// http://code.google.com/chrome/extensions/browserAction.html#event-onClicked
chrome.tabs.getSelected(null, function(tab) {
  chrome.tabs.sendRequest(tab.id, {greeting: "hello"}, function(response) {
    document.getElementById('response').textContent = response.farewell;
  });
});
</script>
Hello, world!
<p>Response: <span id=response>Awaiting response...</span>