Changing the CreateSite page

After seeing the templateID detail ( ChrisF wanted to know if there was an ID associated with the Custom Tab that shows up on the Create New Page.  He was trying to make that tab the default tab that shows up.  Here is my response:

If you look at ...\12\TEMPLATE\ADMIN\createsite.aspx, you'll see that dialog is simply represented by:
    <wssuc:TemplatePickerControl id="InputFormTemplatePickerControl" runat="server" ShowSubWebOnly="false" ShowCustomTemplates="false" />
Look at the definition of wssuc:TemplatePickerControl in the Register directives at the top of the page and you'll find that it is comming from ~/_controltemplates/TemplatePickerControl.ascx.  Go open that (its in: ...\12\TEMPLATE\CONTROLTEMPLATES) and you'll see this:
    <input type="hidden" ID="HiddenSelectedCategory" value="" runat="server"/>
You might think that you could make this something like:
    <input type="hidden" ID="HiddenSelectedCategory" value="Publishing" runat="server"/>
however, the hidden tag is used only on postback--so it won't help you on page load.

Your only real answer here is to use querystrings on the inbound URL.  If you browse to createsite.aspx with a DisplayCategory querystring, the control will filter.  For example:
Alternatively, if you browse to createsite.aspx with an ID querystring, the control will also filter.  For example:
will just show MPS configurations, and:
will show the Collaboration Portal as the only choice.

Now your next question (and what ChrisF was getting at) is--how do I make the UI do this?  The answer, edit the ...\12\TEMPLATE\FEATURES\AdminLinks\Applications.xml
Look for CreateSite, you will see:
           Url="/_admin/createsite.aspx" />
Make this (for example):
           Url="/_admin/createsite.aspx?DisplayCategory=Publishing" />

Save it and refresh the page.  You'll see the link is now updated.  
Now I have to throw out the disclaimer that this modification may need to be redone/removed with a servicepack, hotfix, or upgrade to o14 (make sure you let CSS know you made this change if you call with an issue with Central Admin).  However, if you decide to accept the risk associated with this kind of change, yes you can make it happen.