This is pretty cool (linked from /.) GNOME Storage is a module that allows file access through a) sql and b) natural language. So not only do you get to access files in a way such as "is file type of MP3 and artist = 'U2'", but you do it in natural language... "songs by u2". Very more "star trek" than "id3 -l *mp3 | grep -i U2" wouldn't you say? They have some other really cool looking features as well. I'm looking forward to see where this is going, and how similar it is to the all new wonderful doubleplusgood filesystem in the new Windows (Longhorn). I'm thinking that GNOME Storage will be out first though :)
ReiserFS 4 is going more in this direction as well. Ie, much stronger metadata. So strong that it can replace a traditional RDMS. And it's extendible OO nature gives you warm fuzzy feelings.
The main problem (presented by /.) is getting the metadata into the system. Once it's there and accurate you're gold, but getting it there is a bitch. Really only mp3s and a couple of other file types provide information other than what can be gleemed by file(1) via id3 tags, and even just getting the type of file you can only really do for net protocols with http, and that only is as good as the server, which more times than none either relies on the programmer or the file extension to know what it's sending to you. Personally I'd love it if someone would come over and index all my mp3s properly (I'm in the process now and it's tedious to say the least) and all my movies properly (how?) etc.
What files do you want metadata on? I'm betting that mp3 and jpeg already have maetdata, which is probably the bult of most folks files. Seriously look at what you have on you file system right now and what files are missing the metadata?
Relatedly some metadata can be aurired by use, as in the use relationships create metadata. Think something like Google page rank applies to the local storage.