[somat]

Note that "Mickey: the character" does not suddenly become public domain, Mickey is a trademark of the Disney corporation, and trademarks do not expire. However some specific "Mickey: the film" will go into public domain.

I am not a lawyer, but I suspect this means that is will be fine to redistribute specific early mickey films, however any remixes, or reuse of the content, you may find yourself violating Disney's mouse trademark. You would have to successfully argue that your use of Mickey came form a public domain source and does not exist in the same market as Disney's trademark... Good luck on that, you'll need it.

[slim]

Steam boat willie : the character will become public domain. You are free to create new stories with steam boat willie as a character.

[toast0]

I'm not a lawyer either, if copyright is expired, you're free to make a derivative work. Although, you certainly have to be sure you don't derive from a derivative work whose copyright has not expired.

Trademark issues will be tricky, there is a concept of fair use in trademarks, and it seems to me, if you can sell a copy of the film, you must be able to use elements of the film to sell it. If you can create and distribute derivative works, you must be able to use elements of the works to sell them. But maybe we'll just have to have white boxes with 3rd party Mickey (as seen in Steamboat Willie) movies, and boring movie posters too. I expect to see a stream of lawsuits until clear rulings are available.