If you have playlists that are not automatic, not 'smartlists' but that you set up for yourself -- so for ex, rather than setting the playlist as a composer or as a group you make up your own personal score, you must drag and drop into it, it won't automatically update. On Smart lists you set the parameters in advance, like a group's name, or a genre, and it automatically updates any new uploads or downloads on your computer that match that parameter -- whether you want the new stuff in there or not.
BUT if you set a smart playlist by say a specific rule -- for ex, I set one up by 'album is' Dvorak -- and later added new Dvorak pieces from a different album, it won't recognize and add it to the ipod -- you have to change the playlist rule to 'composer is.'
If you switch between computers it will only update automatically off of one computer.