Also it could remain copyrighted for an eternity if a court declares it so (eg. disney has a lot of lawyers on retaining looking after their related works). The corporations want copyright to last FOREVER (well Sonny Bono said forever minus 1 day was ok) and they can throw millions into lobbying and court costs.
See..
http://en.wikipedia.org/wiki/Copyrig..._Extension_Act
and..
http://www.reason.com/news/show/36416.html
In a larger somewhat offtopic context, what scares me the most is that corporations patent natural enzymes occurring in certain plants which has useful effects (medicinal or otherwise), in effect securing intellectual property from nature. Then of course the courts, IP laws, Copyright, Patent laws and the whole machine would give the corporation power to attack people using the natural enzymes from plants in other products. Corporations should not own the world's knowledge and the power of naturally occurring flora.