gistfile1.js

var sticker = $('yourElement');
var origin = sticker.cumulativeOffset();
var width = document.viewport.getWidth();
var offset = 10; //pixels from top
var test = new Element('div',{style:'position:fixed'});
var cantBeFixed = (test.style.position.toLowerCase() != 'fixed');
//or, if you also want to simulate the effect on iOS, uncomment the following line
//var cantBeFixed = (Prototype.Browser.MobileSafari || test.style.position.toLowerCase() != 'fixed');
sticker['_style'] = sticker.readAttribute('style');
function stick(){
  var scrolled = document.viewport.getScrollOffsets()[1];
  if(scrolled >= (origin['top'] - offset)){
    if(cantBeFixed){
      sticker.setStyle({"top":(scrolled + offset) + "px"});
    }else{
      if(!sticker.stuck){
        sticker.setStyle('position:fixed; top:' + offset + 'px; left:' + origin['left'] + 'px;');
        sticker['stuck'] = true;
      }
    }
  }else{
    sticker.writeAttribute('style',sticker._style);
    sticker.stuck = false;
  }
}
Event.observe(window, 'resize', function(evt){
  sticker.writeAttribute('style',sticker._style);
  sticker.stuck = false;
  origin = sticker.cumulativeOffset();
  stick();
});
Event.observe(window,'scroll', function(evt){
  stick();
});