Here's a simple detect function if you want to show adblock users a big paywall, which could be good if you have enough of them.

  function detect(cb) {
    var tt = document.createElement("div");
    tt.style.display = "none";
    document.body.appendChild(tt);
    var i = document.createElement("iframe");
    i.src = "http://adv.foo.com/ads/-adspace?ad_id=&affiliate=&advert=678";
    var foo = i.style.cssText;
    tt.appendChild(i);
    window.setTimeout(function() {
        cb(foo!=i.style.cssText);
    }, 200);
  }

the callback 'cb' will be called with true if adblock is installed.