I agree filesystem versioning is nice, but take a look at VMS, it had this. There are a lot of cavets with this, like the fact that you have to delete old versions to free up disk space, etc. and how many times have you saved a file just because of habit? If it was an optional on, then it might be useful. Though with NetApp snapshots and snapshots supported under SoftUpdates, you aren't to far from supporting file system versioning.
A friend of mine mentioned that you might want to check out BeOS for filesystem metadata. Sounds like you can have "virtual folders" that contain queries of the meta-data. So you can have a folder of all of your mp3's or other such things. I haven't ever used BeOS myself, but probably something to checkout.