The most important pieces of their intellectual property are the trademarked characters, and trademarks never expire; they could probably allow the earliest works to fall under public domain and still maintain a large degree of control.

There are a little over a dozen influential Superman cartoons[1] from the 1940s that are public domain. That's the kind of thing that would be public domain -- it couldn't be a Mickey merchandising free-for-all or anything.

That being said, they certainly won't give up copyright protection unless they are forced.

[1] http://en.wikipedia.org/wiki/Superman_(1940s_cartoons)