Here's a solution that we ended up working with (in order to deal with some edge cases and older versions of Internet Explorer, we eventually also faded out the title bar on scroll then faded it back in when scrolling ends, but in Firefox and WebKit browsers this solution just works. It assumes border-collapse: collapse.
The key to this solution is that once you apply border-collapse, CSS transforms work on the header, so it's just a matter of intercepting scroll events and setting the transform correctly. You don't need to duplicate anything. Short of this behavior being implemented properly in the browser, it's hard to imagine a more light-weight solution.
JSFiddle: http://jsfiddle.net/podperson/tH9VU/2/
It's implemented as a simple jQuery plugin. You simply make your thead's sticky with a call like $('thead').sticky(), and they'll hang around. It works for multiple tables on a page and head sections halfway down big tables.
$.fn.sticky = function(){
$(this).each( function(){
var thead = $(this),
tbody = thead.next('tbody');
updateHeaderPosition();
function updateHeaderPosition(){
if(
thead.offset().top < $(document).scrollTop()
&& tbody.offset().top + tbody.height() > $(document).scrollTop()
){
var tr = tbody.find('tr').last(),
y = tr.offset().top - thead.height() < $(document).scrollTop()
? tr.offset().top - thead.height() - thead.offset().top
: $(document).scrollTop() - thead.offset().top;
thead.find('th').css({
'z-index': 100,
'transform': 'translateY(' + y + 'px)',
'-webkit-transform': 'translateY(' + y + 'px)'
});
} else {
thead.find('th').css({
'transform': 'none',
'-webkit-transform': 'none'
});
}
}
// See http://www.quirksmode.org/dom/events/scroll.html
$(window).on('scroll', updateHeaderPosition);
});
}
$('thead').sticky();