gistfile1.js

var sticker = $('cart');
var origin = sticker.cumulativeOffset();
var offset = 10;
sticker['original'] = sticker.readAttribute('style');
Event.observe(window,'scroll', function(evt){
  var scrolled = document.viewport.getScrollOffsets()[1];
  if(scrolled >= (origin['top'] - offset)){
    if(Prototype.Browser.IE){
      sticker.setStyle({"top":(scrolled + offset) + "px"});
    }else{
      sticker.setStyle('position:fixed; top:' + offset + 'px; left:' + origin['left'] + 'px;');
    }
  }else{
    sticker.writeAttribute('style',sticker.original);
  }
});