Shameless plug, and hopefully a few answers from a Sprite employee
Actually Playlists are stored on your device in a database,
They are independent of the media on your SDCard
Music files, albumn art etc are stored on the SDCard.
Try Sprite Backup, it backs up and restores music, playlists, art etc from a single backup file
We were the first to do it, as with many other content.
Problem with backing up only playlists, and not the media files, your playlist backup will get out of date with the actual media files oncard... Also as many people know SDCards do fail (hence a reason to backup), so a single backup file, easy to copy off of SD is a great way to go.
Sprite Mobile - Android Backup - SMS, Bookmarks, System Settings, Application Settings, Call Logs, MMS, Contacts, Photos, Video, Music and much more...