upgrading to 5.0.1 might have caused this to happen to me. i just upgraded this past week when i switched iphones.... so maybe the switch to a new phone, and then restoring from icloud had something to do with it also.
This just happened to me, the same as the original poster, with one exception.
I don't use itunes in the cloud or music match. don't have either one setup. the cloud stuff might be automatic for all i know, but i know i don't have music match.
anway, ONLY the playlist i sync to the phone have been emptied. This either has to do with i updated iTunes, or because i just switched iphones last week. switched from one 4s to a different 4s. i switched ipads also (same model, just a switch). or because the new iphone has 5.0.1 on it when i had not upgraded the previous one. something causes my playlists to be emptied....
HOW DO I GET THEM BACK?
I know itunes keep this "previous itunes library" files in the music folder. can i somehow use that to be my plastlists back????