Public domain

[Stefan Persson]

This has been discussed here before, but when does something become public
domain?  I have in my hand a Japanese DVD edition of The Three Caballeros (三人
の騎士 or Sannin no Kishi) with no Disney logos or copyright information
anywhere, except for a big notice where it says that the movie belongs to the
public domain.  How is this possible?  I thought that Disney still owned several
trade marks related to the movie (such as the very commonly used Donald Duck and
José Carioca trade marks).

I haven't seen any Disney-related products before where it says that the product
belongs to the public domain.

Stefan