According to this thread on the official LittleBigPlanet forums, user-created levels are being deleted from the game's servers without warning over copyright issues.
Levels cited in this Destructoid piece contained elements of the Metal Gear Solid and Sonic IPs. Apparently, there's no going back once the level's deleted, either: once it's gone it's gone. Not so great if you've really put the work in.
More through the links.