Although this is a rather old question. MSIE has a feature that allows scrolling to hide when not in use. This is very similar to the OSX lion scrollbars. Plain CSS, NO JS.
body {
scrollbar-base-color: #222;
scrollbar-3dlight-color: #222;
scrollbar-highlight-color: #222;
scrollbar-track-color: #3e3e42;
scrollbar-arrow-color: #111;
scrollbar-shadow-color: #222;
scrollbar-dark-shadow-color: #222;
-ms-overflow-style: -ms-autohiding-scrollbar;
}
MSIE, , , -ms-authiding-scrollbar.