I just found this blog entry that describes a way to generate a smart playlist that will contain music that you like, but mix in music that you haven't heard in a while. Also this playlist prioritizes music that was just added to your library.
I actually like the results that the playlist returns. The only problem that I have is that my "Expert Playlist" only has about 170 tracks. But, that is probably caused because I haven't rated enough tracks.
I would like to see something like this in Netjuke, so it can be used in Netjuke TiVo. The problem is that the Netjuke database does not the last play date or ratings for each track. Maybe the new version of Netjuke will contain these fields.