This can actually be done without changing the core.

What you can do is create a SmartSkin™ which has logic in it to load other skins based on the Portal Alias.

I have created these kinds of skins for the SkinCovered.com subscribers that allowschanging the whole skin based on the time of day, and based on the Role of the user.

I'll go ahead and do one for this task and release it for free.

We forked from the core at 3.2 using the subportal module originally developed by Tom Mahoney / ezcam (Available on snowcovered).  The module not only allowed us to skin at the portal alias level but also extended some of  the core modules allowing for alias specific content. and subportal admins.  (Limited to just the modules and content we designate)

ie. Sub Contacts, Sub html, Sub image, Sub faq's, Sub feedback, Sub registration.  Relatively simple concept: Associate subportal ID to a portal alias and use the subportalid to identify the module content to be delivered.  In effect we have global and subportal versions of modules.  Global shows on all aliases and subportaled delivers content specific to a portal alias.  Very fast and very slick application. 

What this provided us was a managed content application engine that we utilized to build industry specific portal applications.  ie. computer reseller store sites, branded affiliate sites.  Each with their own look and feel but with professionally managed content.

Our fork allowed us to install multiple hosts in a single DB each with  multiple portal applications  and  with branded  sub sites in a single DB.    We actually setup a proof of concept test site on a $10 / month wh4l account with a single DB running 5 DNN host installations each with 10 portal applications and 10,000 dynamically branded subportals.  The performance was amazing since the framework was really only serving 50 optimized portals with managed content.   There were some minor mods made to the core and admin through hosts - portals was painful (Portals and aliases have no paging.... try pulling 100,000 records ;).  But it worked very well for us  if the site is still up I'll post a link.

The current revision of the framework has changed dramatically and we are trying to update the engine for this style of functionality.  Beyond portal alias skinning we need the ability to deliver alias specific content and allow for alias specific administration.  This really should be part of the core framework...  Look at Cafepress, Community Zero, myspace etc.

Fully Managed content applications is a huge value add that end users have no problem paying $$$ for especially when it reflects their brand!

This is how I do it. All you need is the following code pasted into a file by itself.  Then you can name it something like "PortalAliasSwitcher.ascx" and put it in a skin folder along with the skins that you want it to load based based on the Urls.  Then go in and assign this file to the page, site, or host settings like you would normally do an individual skin.

[Code]

<%@ Control language="vb" AutoEventWireup="false" Explicit="True" %>

<div id="ControlPanel" runat="server" />

<asp:placeholder id="SkinPlaceholder" runat="server" />

< s c r i p t runat="server">
    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    '**  Skincovered.com Portal Alias based skin switcher
    '**  Switches the skin depending on the Url used to access the site
    '**  Select this skin (PortalAliasSwitcher) as would any other skin in DotNetNuke
    '**  The code below will load a skin from the same folder depending on the Url
   
        Dim skin As String = ""
        Select CType(HttpContext.Current.Items("PortalSettings"),PortalSettings).PortalAlias.HTTPAlias.ToLower()
            Case "hr.mydomain.com":
                skin = "HumanResources.ascx"
            Case "it.mydomain.com":
                skin = "InformationServices.ascx"
            Case Else:
            'this skin will be loaded if no matches were made to the portal aliases above
                skin = "default.ascx"
        End Select
       
        'this line loads the actual skin.
        SkinPlaceholder.Controls.Add(LoadControl(skin))
    End Sub
</ s c r i p t>
[/Code]