I found a hidden Spotlight feature accidentally while using the Migration Assistant. I noticed that all my "incoming" files were saved into a folder called "username.noindex" ... and Spotlight did not try to look into these folders.
This works with every folder. So if you want to create some Spotlight excluded folders, just add ".noindex" to the folder's name -- "blind.noindex," for instance, to create some "blind spots."
[robg adds: You can do the same thing, of course, using the Privacy tab in the Spotlight preferences panel. This method seems quicker, though I don't know how it works for content that's already been indexed -- you might need to add then remove that folder from the Privacy tab anyway to get the index to be updated...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050527103342870