No, I'm logged in to the domain with my own id (addomain\lacht) but I am logged in to the portal as web admin (addomain\webadmin).  I put a page Test.aspx in the <DNN_Install>\Child folder with the contents listed below, and it in fact shows that LOGON_USER is ADDOMAIN\WebAdmin.  So (as I'm sure you already knew ;-) the authenticator is working properly, though the end result is not what I want.  I'll need to look in a different direction for the solution to this issue.

<%@ Page language="VB" %>
<TABLE>
      <TR>
           <TD>
                <B>Server Varriable</B>
           </TD>
           <TD>
                <B>Value</B>
           </TD>
      </TR>

      <% For Each name as String In Request.ServerVariables %>
      <TR>
           <TD>
                <%= name %>
           </TD>
           <TD>
                <%= Request.ServerVariables(name) %>
           </TD>
      </TR>
      <% Next %>
</TABLE>

I had done so at one point but removed it.  It works fine for another person in my group, so it's something specific to me.  I'm guessing that it might be because I've used a link to that machine's drive (\\machine\c$) as the webadmin user and told it to save the password.  It's an internal web site so we hit it as http://machine/sitename.