I think the title on this one says it all. Say you have a collaboration portal with a search center in a site collection while a records center sits beside it in the same web application. The goal is to hide the records center from the collaboration portal's search, while maintaining search inside the records center.
We know that we can use architecture, multiple SSPs and Web Applications, etc. However, Jingmei found a much simpler approach documented here:
Add a rule on the "All Sites" scope within the SSP to remove the site(s) from the enterprise search.
Thanks Jingmei and Sid!