I believe the only way to accomplish this would be to 'audientize, the
top link bar  headings/links using AD Security groups or SSP Global
audiences.

But how do I do that? Is that some setting  in
SharePoint or some code that I need to do?

Sites that are added to the top  nav bar  when they are created  are
security trimmed. sites  that are added manually later are not security
trimmed. You can see the difference in the database if you look, but I
don't know any way to add a security trimmed site  tab manually. Only at
creation time.

I did not know that native site  tabs were security
trimmed.

To trim sites/links that are manually added requires SharePoint Server
Publishing Features, and a Shared Services Provider if you want to use
complied audiences (faster than AD group targeting). You must enable
publishing features for the site and:

1. Create a global Audience in Shared Services Administration or
use an existing AD Security Group (global)

2. Create the tab in Site Settings, Navigation, Global navigation

a. During creation, there is a section at the bottom to define the
audience