Thanks Tony, I did already know about those. They look cool, but I don't really use them because:

1) My playlists define my categorization, not the Genre tag. I want to use any given level of the hierarchy, which varies by song and my mood, rather than a flat selection based on matching only one tag
2) I have to keep pressing the remote button, once per song to add, I want to press once to select the whole playlist
3) I don't want to have to use the remote in the car while I'm driving

It seems that it should be an easy addition to the UI, just an easier/more context-sensitive way of accessing the playlist menu structure