It seems like that's only possible now when importing/ripping CDs. For existing tracks, it doesn't work.

However, you can right click a group of tracks, then edit the info to add a unique Grouping label. This allows you to shuffle by grouping.

In fact, iTunes purchases - at least some of them - already include this. Screenshot: http://cl.ly/image/3w0R0y1I0E0V

How, though? As discussed elsewhere in the thread, it doesn't seem as though "smart playlists" are quite that smart, and I'm not familiar enough with iTunes to make a guess as to how else it might be accomplished.