worker.js

onmessage = function(event) {
  postMessage(event.data);
  onCancel(); // causes "onCancel is not defined" error
}

workerterminate.user.js

// ==UserScript==
// @name         GM_woker test 4
// @namespace    gWorkerTest
// @include      *
// @version      0.1
// @author       Erik Vergobbi Vold
// @license      GPL version 3 or any later version; http://www.gnu.org/copyleft/gpl.html
// @resource     worker http://gist.github.com/raw/625195/worker.js
// @description  A GM worker test.
// ==/UserScript==

alert('making worker');

var worker = GM_worker("worker");

worker.onerror=function(event) {
  alert(event.message);
};

alert('posting msg');
worker.postMessage('hello world!');

alert('done');