You can do this already
I've got top level playlists of Artists, with their Album's as sub playlists.
eg,

The Egg -> Albumen    -> Albumen Tracks
        -> Travelator -> Travelator Tracks

In this way, I can play All by an Artist or just an Album.
I also have a top level Item Called Techno, with Albums by Juno Reactor, Eat Static etc in it, so I can play a whole Genre =)

Jazz
(List 112, S/N 00030, 4 gig blue)