I know in previous versions of DotNetNuke you couldn't nest skins without implementing some sort of hack like a Sever Side Include such as "<--#include file=”includes/header.ascx” -->". 

For example if I had an Index.ascx skin I couldn't create a Header.ascx skin and then "import" that Header.ascx skin into all my other skins.  The reason the following doesn't work 100% is that it complains about the <%=SkinPath%> in the Header.acsx because it is a code block that is nested within a an element that has a "runat=server" (<include:Header runat="server" />) attribute.

Something like the following in the Index.ascx page:

<%@ Register tagprefix="include" tagname="Header" src="~/Portals/_default/Skins/PositiveTomorrows/includes/Header.ascx" %>
<div id="wrapper">
<include:Header runat="server" />
</div>

And then something like this in the Header.ascx page:

<%Control Language="C#" Inherits="DotNetNuke.UI.Skins.Skin"%>
<%@ Register TagPrefix="dnn" TagName="LOGIN" Src="~/Admin/Skins/Login.ascx" %>
<div id="header">
<img src='<%=SkinPath%>/images/logo.png' alt="" />
</div>
</div>

Has a way to do this been added in DotNetNuke 6?