[Setup] How to exclude a directory from search?

[David Kurtz]

I'm building a site for a client who wants a 'files' directory which  
is invisible to search, but whose contents are available to everyone,  
including anonymous users.

I tried to accomplish this by customizing queryCatalog, since that  
seems to be the mechanism search was using, but it also broke folder  
views for that files directory.

It seems like this would be a commonplace practice for certain types  
of site, but I can't find any documentation, and I'm not sure where  
to start probing inside search and portal_catalog.

Thanks for any help.