I just realized that I've been remiss and have yet to mention Ben Summers' SpotMeta.
Remember when I was grousing about the disconnect between Spotlight and HFS Extended Attributes? SpotMeta fixes this disconnect.
SpotMeta allows you to set pretty much any arbitrary name/value pairs on any file, to be indexed and searchable by Spotlight. Furthermore, it provides GUI tools and Finder integration to make it all usable in practical day-to-day work.
I haven't had time to spend playing with it much or to figure out exactly how it works, but I think the possibilities are just beginning for this fine hack. Will it realize the promise I saw in BeFS for a filesystem/database hybrid?