We have an enterprise portal and we use a GPO to make the Portal Top Level site
the default Home Page for all users. Each department has a site  under the Site
Directory.

I like the idea of using the top  level site  as the corporate home page - my
company does this and we also push it out as the default home page for all
users. Most users  know how to change the default, so we have to invent ways to
get them to come to the page. Things of interest to all employees - navigation
to benefits information, electronic time cards, 401k or stock info, these are
all good. But not really interesting enough. Does your location have a
cafeteria? Post today's menu. Is the local sports team in the playoffs? Post the
scores, stats, links to highlights videos. Does your CEO have a blog? Capture
the latest post and his/her photo/video. Incorporate navigation and tagging, a
search with multiple scopes,... if it's interesting and/or useful then they will
come. Then you can post announcements  & news with some small  amount of
confidence that your users may actually see them.

We are working on such ways
to create a very compeling reason to visit the top  site. If we are
successful, I'll post what we did.