jQuery saves the day... again!
CSS:  
#wrapper {
  position: absolute;
  width: 200px;
}
#fancyDiv {
  position: absolute;
  top: 0;
}
#fancyDivt.fixed {
  position: fixed;
  top: 0;
}
html:
<div id="commentWrapper">
  <div id="comment">
    <p>some text</p>
  </div>
</div>
jQuery:
$(document).ready(function() {
    $(function () {
        var top = $('#fancyDiv').offset().top - parseFloat($('#fancyDiv').css('margin-top').replace(/auto/, 0));
        $(window).scroll(function (event) {
              // what the y position of the scroll is
              var y = $(this).scrollTop();
              // whether that's below the div
              if (y >= top) {
                // if so, ad the fixed class
                $('#fancyDiv').addClass('fixed');
              } else {
                // otherwise remove it
                $('#fancyDiv').removeClass('fixed');
              }
        });
    }
});