Eldred's "Free the Mouse" slogan refers to how these extensions have kept Mickey Mouse regularly more than 20 years from expiring since 1960 (see selfsame "forty years" item). Lessig often states that "no one can do to the Disney Corporation what Walt Disney did to the Brothers Grimm"--that is, improve on our free cultural heritage (a valuable service, as indicated by Disney Corporation having enough money to buy the continued monopoly on Walt's inventions). You may have seen Steamboat Willie, the first Mickey Mouse cartoon and so the item being kept in copyright, but Lessig has taken pains to point out it's a spoof on a Buster Keaton film.
What can people do with public domain material? Make comic books. Build services like the ones Scott Andrew LePera uses to publish his band's music. Build the electronic library. DJ, and build the World Wide Web, as Matt Haughey describes. Be the next Walt Disney.
Doubtless Aaron Swartz, invited by Lessig to attend, will have things to say afterward. Matt Haughey wrote, as I said; he built and runs MetaFilter, but he's also been working with Lessig's Creative Commons project to build an index of public domain material. Yale's LawMeme has a two part series dissecting Eldred's brief for programming-minded laymen.
Best of luck to Eldred and Lessig.
Utterly unrelated links: The 1 percent solution of paid web content. Statistical laws of usability. Peter Van Dijck's usability weblog. WaSP's weblog has RSS. Burningbird's The parable of the languages, a programming philosophy piece.