It's the wave of the future - file your patents now. ;)
It doesn't have file system integration, but from what Jobs was hinting at during the last WWDC, something like that is on the table, FWIW.
There was a KDE-based metadata filesystem thing that got some play on /. some time ago. Don't recall what the name of it was - I followed it for a while until the author decided to call it quits. Good stuff, tho.
On a different note, Opera's mail-client-formerly-known-as-M2 operates on a database-like interface. It's a real love-hate thing, but I like it quite a bit.
Ack. FOSS content of post: 03%.