Illegal File Formats?

Patman asks: "While thinking about the legality of MP3's today, I got to wondering - has a file format or storage method ever been declared illegal? Of course, using it can be illegal if you don't have rights, but I was wondering about actual formats or methods. " I don't think this has happened yet, but don't think that it never will. I wonder if I someone will start a betting pool on which one it will be.