iTunes 4.2 adds a "Grouping" field on the tag screen, which sounds interesting, but it's currently undocumented. I did a Feedster dig and found this thread on Apple's discussions board. Sounds interesting -- some folks are using it as a subgenre field, others have different applications. Well, more metadata to key smart playlists on is always a nice thing. One wacky thing is that it seems to have picked up the Emusic classifications scheme (for tracks I downloaded from that service), which implies that whatever field ID3 Apple is calling "Grouping" is being used for other things by other people.
I’ve given up reading books; I find it takes my mind off myself.