I have a number of playlists that I use strictly for administrative purposes, such as presenting all unrated songs for rating, songs tagged as temporary, etc. To distinguish between these and playlists used to find music for playing, I put a bullet (option-8) and a space in front of the playlist name. In this way, the administrative playlists get grouped at the beginning of the list of sources in iTunes and playlists on the iPod. It's easy to distinguish between playlist types in this way.
This is a great idea! I'm waiting on nested playlists.. now that would be even better.