Some code I modified from sopablackout.org to fit the needs of the Open Source Club. It temporarly blacks out the page until the user clicks. This however could be used for any type of important webpage announcement. You should probably talk to sopablackout.org for rights to this code before using it for any other purpose.

(function (){
 var root = this;

 var SopaBlackout = function(){};
 var addEvent = function(obj, type, fn, ref_obj){
  if (obj.addEventListener){
   obj.addEventListener(type, fn, false);
  }else if (obj.attachEvent){
   obj["e"+type+fn] = fn;
   obj[type+fn] = function(){
    obj["e"+type+fn](window.event,ref_obj);
   };
   obj.attachEvent("on"+type, obj[type+fn]);
  }
 };
 // Thanks http://javascript.nwbox.com/IEContentLoaded/
 // for this
 var IEContentLoaded = function(w, fn) {
  var d = w.document, done = false,
  init = function () {
   if (!done) {
    done = true;
    fn();
   }
  };
  (function () {
   try {
    d.documentElement.doScroll('left');
   } catch (e) {
    setTimeout(arguments.callee, 50);
    return;
   }
   init();
  })();
  d.onreadystatechange = function() {
   if (d.readyState == 'complete') {
    d.onreadystatechange = null;
    init();
   }
  };
 }
 var onDomReady = function(fn){
  if (document.addEventListener){
   document.addEventListener('DOMContentLoaded', fn, false);
  }else{
   IEContentLoaded(window, fn);
  }
 };
 var getStyle = function(e, prop){
  if (e.currentStyle){
   return e.currentStyle[prop];
  }else if (document.defaultView && document.defaultView.getComputedStyle){
   return document.defaultView.getComputedStyle(e, "")[prop];
  }else{
   return e.style[prop];
  }
 };
 var findPos = function(obj){
  var curleft = 0;
  var curtop = 0;
  if (obj.offsetParent){
   do{
    curleft += obj.offsetLeft;
    curtop += obj.offsetTop;
   }while(obj = obj.offsetParent);
  }
  return [curleft, curtop];
 };
 var txt = function(s){
  return document.createTextNode(s);
 };
 var create = function(e, props){
  var elem = document.createElement(e);
  var props = props !== null ? props : {};
  for (var key in props){
   if (key == 'href'){
    elem.href = props[key];
   }else if (key == 'src'){
    elem.src = props[key];
   }else{
    elem.style[key] = props[key];
   }
  }
  l = arguments.length;
  for (var i=2; i<l; i++){
   elem.appendChild(arguments[i]);
  }
  return elem;
 };
 var getOpts = function(){
  var ret = {};
  for (var key in SopaBlackout.DEFAULTS){
   var k = 'sopablackout_' + key;
   ret[key] = (typeof window[k] === 'undefined') ? SopaBlackout.DEFAULTS[key] : window[k];
  }
  return ret;
 };
 var dateMatches = function(spec){
  spec.push(false); spec.push(false); spec.push(false);
  var today = new Date();
  if ((spec[0] !== false && today.getFullYear() !== spec[0]) ||
    (spec[1] !== false && today.getMonth() + 1 !== spec[1]) ||
    (spec[2] !== false && today.getDate() !== spec[2])){
   return false;
  }
  return true;
 };

 SopaBlackout.VERSION = '0.2.0';
 SopaBlackout.MIN_HEIGHT = 100;
 SopaBlackout.HEADER_TEXT = "This is what the web could look like under the Stop Online Piracy Act.";
 SopaBlackout.CONTINUE_TEXT = "(click anywhere to continue)";
 SopaBlackout.ZINDEX = Math.pow(2, 31) - 2;
 SopaBlackout.DEFAULTS = {
  'id': false,
  'srsbzns': false,
  'on': false
 };
 SopaBlackout.blackout = function(opts){
  var obj;
  var body = document.body;
  if (opts['id'] === false){
   obj = body;
   height = "100%";
  }else{
   obj = document.getElementById(opts['id']);
   var height = parseInt(getStyle(obj, 'height'), 10);
   height = height > SopaBlackout.MIN_HEIGHT ? height : SopaBlackout.MIN_HEIGHT;
  }
  var offsets = findPos(obj);

  var blackout =  create('div', {
      top: offsets[1],
      width: '100%',
      backgroundColor: 'black',
      zIndex: SopaBlackout.ZINDEX,
      height: height,
      color: '#999', padding: '100px', color: 'grey', fontfamily:'Georgia,Times,serif', lineheight: '1.5',position: 'absolute',},

 create('img', {src: "open_source_banner_2.png", backgroundcolor: "#000"}, txt("")),
 create('h1', {fontweight: 'normal', letterspacing: '3px', margintop: '3em'}, txt("THIS SITE IS ON STRIKE")),
 create('div', {width: "450px"}, txt("Like Wikipedia and many other sites, this site is 'blacked out' for 24 hours to protest two bills before the United States Congress, known as SOPA and PIPA."),create('br'),txt("These bills endanger free speech in the United States and abroad, setting a frightening precedent of Internet censorship for the world.")),
 create('ul', {width: '435px'},
  create('li', null,
   create('a', {href: "http://blog.reddit.com/2012/01/technical-examination-of-sopa-and.html", textdecoration: "None", color: "#990000"}, txt("A Quick Overview of SOPA and PIPA")),
   txt(" (see "),
   create('a', {href: "https://en.wikipedia.orgwiki/Wikipedia:SOPA_initiative/Learn_more", color: "#990000"}, txt("Wikipedia")),
   txt(" for details)")
  ),
  create('li', null,
   txt("Take action in the US: "),
   create('a', {href: "http://sopastrike.com/strike/", color: "#990000"}, txt("online")),
   txt(", "),
   create("a", {href: "http://americancensorship.org/modal/call-form-moz.html", color: "#990000"}, txt("by phone")),
   txt(", or "),
   create("a", {href: "https://www.eff.org/deeplinks/2012/01/stop-blacklist-legislation-guide-person-meetings", color: "#990000"}, txt("in person"))
  ),
  create('li', null,
   txt("Take action anywhere: tweet about #SOPASTRIKE, "),
   create('a', {href: "http://www.blackoutsopa.org/", color: "#990000"}, txt("wear a badge")),
   txt(", join with your own site or blog")
  )
 ),
 create('div', {letterspacing: "1px"}, txt("PLEASE HELP TO PROTECT FREE SPEECH ONLINE."))

  );
  if (opts['srsbzns'] !== true){
   blackout.appendChild(create('p', {paddingTop: '250px', color: '#333'}, txt(SopaBlackout.CONTINUE_TEXT)));
   addEvent(blackout, 'click', function(e){
    body.removeChild(blackout);
   });
  }
  body.appendChild(blackout);
 };
 SopaBlackout.go = function(){
  var opts = getOpts();
  if (opts['on'] !== false && !dateMatches(opts['on'])){
   return;
  }
  SopaBlackout.blackout(opts);
 };

 onDomReady(SopaBlackout.go);
}).call(this);