I'm working on a super secret project for Hydrant at the moment which requires vertically centering an element on page load and keeping it centered on window re-size. Here's how it's done.

$(function() {
  $.fn.verticalCenter = function() {
    $(this).each(function() {
      $pageCenter = $(this).height() / 2;
      $windowCenter = $(window).height() / 2;
      $top = $windowCenter - $pageCenter;
      $(this).css({
        'position': 'absolute',
        'top' : $top + 'px'
      });
    });
  }
  myResize = function() {
   $('.page:visible').verticalCenter();
  }
  $(window).resize(myResize);
  myResize();
});