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.
Parallel lines never meet, unless you bend one or both of them.