The Magic Kingdom is 'not threatened'
In a surprise decision, the court has said it will examine the legality of the 1998 Sonny Bono Copyright Act - named after the late singer and Congressman - which added 20 years to all existing copyrights.
"We figure that Thomas Jefferson had it right when he insisted that the US constitution, which provides for the existence of copyright amongst other things, said it had to be for a limited time," Jonathan Zittrain, one of Mr Eldred's lawyers, told the BBC's World Business Report.
"He (Jefferson) was worried people would keep extending the term and that's exactly what is happening right now and we figure the constitution has protected against."
US Solicitor General Theodore Olson, arguing for the government, supports the law and in a brief said it does not conflict with any legal precedents.
Mickey Mouse case
A reversal by the court of the law would free up thousands of copyrighted works, including early films, by making them public domain.
"The magic of the public domain as a concept is that after a certain period of time, certain kinds of works are for all time completely free to use or misuse," said Mr Zittrain.
There is a lot at stake for companies like Disney, which backed the original legislation and whose exclusive rights to Mickey Mouse expire in 2003.
"(Disney's) worry is vastly overblown. They'll still have trademark over Mickey Mouse, and that trademark lasts as long as Disney and the Magic Kingdom do," said Mr Zittrain.
Overstepped
"Even if we win, people could not start opening competing amusement parks with Mickey Mouse as the mascot, that's protected by trademark," he said.
"It does mean that works Disney created decades and decades ago, do become free and ... they can't milk the old stuff for ever."
Mr Eldred, founder of Eldritch Press, publishes old works, like Henry James novels, and makes them available to the world at large in HTML format.
He claims that, in approving the Sonny Bono Act, Congress overstepped the constitution's "plain and express intent to restrict the duration of monopolies over speech".