I assume that you are aware of The permissions that can be assigned to modules? You can set entire modules to be visible/not visible to any security role, including logged in (authenticated) users. So, the easiest way to do this in DotNetNuke is to place two modules where you want that sort of content. Set one to be visible to authenticated users and one visible to unauthenticated users.
That said, you could build something like that into your skin using something like:
<% If Request.IsAuthenticated Then%> <div class="dnn-logged-in"> <%% End If&%%>
rest of your skin
<% If Request.IsAuthenticated Then%> <div> <%% End If&%%>
It it were me, I'd use the tools already built in ...