As you all know, copyright terms have been steadily lengthened via congressional action. Currently, the term is the life of the author plus 70 years. For works authored by corporations—Superman, Mickey Mouse, etc.—the term is 95 years. Thanks to a retroactive clause passed in 1976, the magic cutoff year for corporate creations is currently 1922. Anything published in 1922 or before is in the public domain. Anything after that is still under copyright.
So what happens in 2018? That's only five years away! Well, it's 95 years from 1923, which means that works published in 1923 fall out of copyright. Every year after that, more and more old works enter the public domain. And in 2023 the boom falls: Mickey Mouse will no longer be under copyright.
Will Disney put up with this? Or will they team up with the usual suspects to get the term of copyright extended even further? Tim Lee gives us the lay of the land here.
UPDATE: Sorry, but I bolloxed up the explanation of why 1922 is the current cutoff year for copyright. It's fixed now.