So, on the surface, this did work.
I'm using Visual Web Developer 2008 Express and if I rebuild the website, I get "unknown server tag" errors.
For the code below, I'm getting the errors:
Unknown server tag 'dnn:STYLES'.
Unknown server tag 'dnn:USER'.
Unknown server tag 'dnn:LOGIN'.
----- index.ascx -----
<%@ Control language="vb" AutoEventWireup="false" Explicit="True" Inherits="DotNetNuke.UI.Skins.Skin" %>
<%@ Register TagPrefix="dnn" TagName="STYLES" Src="~/Admin/Skins/Styles.ascx" %>
<%@ Register TagPrefix="dnn" TagName="USER" Src="~/Admin/Skins/User.ascx" %>
<%@ Register TagPrefix="dnn" TagName="LOGIN" Src="~/Admin/Skins/Login.ascx" %>
<!--#include file="page_header.ascx" -->
<div id="ContentPane" class="ContentPane" runat="server"></div>
----- page_header.ascx -----
<dnn:STYLES runat="server" id="ie6fail" Name="ie6fail" StyleSheet="assets/css/ie6.css" Condition="LTE IE 6" UseSkinPath="true" />
<dnn:STYLES runat="server" id="ie7fail" Name="ie7fail" StyleSheet="assets/css/ie7.css" Condition="IE 7" UseSkinPath="true" />
<dnn:STYLES runat="server" id="print" Name="print" StyleSheet="assets/css/print.css" UseSkinPath="true" />
<dnn:USER runat="server" id="dnnUSER" CssClass="user" /> | <dnn:LOGIN runat="server" id="dnnLOGIN" CssClass="user" />
<div runat="server" id="ControlPanel" ></div>