My wife and I share our iPod. It became very annoying to always have to log into my account to update the songs and playlists. I put my library file in a shared folder and then made aliases to the library file and put them in my iTunes folder and my wife's iTunes folder. My expectation was that at least we would then share the same playlists, although I expected that we would still have to be logged into my account to update the iPod. I was pleasently surprised when I logged in with my wife's account and the iPod synched perfectly, no questions asked.
kman