So... I just noticed this problem. I've never had issues syncing my music before but recently I changed the way my playlists are made and today I wanted to listen to one particular ("Rock") playlist... I go to Ipod -> Playlista and it's just not there. The songs are all on the iphone, but the playlist isn't.
In fact, I tried deleting all my music and synchronizing just that one playlist: Again, the songs are on the iphone but the playlist is nowhere to be found.