It seems that the behavior of Smart Playlists with the "Live updating" option disabled has changed in iTunes 8. Specifically, it used to be that you could force these playlists to update simply by opening the "Edit Smart Playlist" window for the list and clicking OK. As of iTunes 8, this no longer updates the playlist. You must instead open the window, select "Live updating", click OK, open the window again, unselect "Live updating", and finally click OK again.
This is proving bothersome to those (like myself) who use a non-live-updating playlist to listen to unrated tracks, rating them as they go. Allowing the playlist to update live interrupts playback whenever you rate a track you are listening to. With the change in iTunes 8, getting a new batch of tracks to rate is a relatively lengthy process.