For clarification...
The only reason Siegel's estate had a case is because Superman wasn't a work-for-hire creation. Siegel & Shuster signed a contract with National Periodicals (Later DC) which only granted DC 28 years of copyright ownership, with the right to one 28 year extension.
The Copyright Act of 1976 added a clause which prevented the terms of existing contracts from being extended retroactively, allowing creators (or their estates) the right to terminate copyright agreements when the copyright term would have originally ended, had Hollywood not been writing legislation with their checkbooks.
Siegel's estate now owns 50% of the American copyright (DC still owns the international copyright) to Superman's story in Action Comics #1. Shuster's estate can petition the courts for the other 50% in 2013.
Because of the "trademark loophole", as long as DC maintains the trademark on Superman, it would be difficult for Siegel & Shuster to do much with their copyright.
DC still owns the copyright on all of the remaining Superman stories. The original story only has Superman (in his original costume) Clark Kent, The Daily Star, a reporter named Lois (no last name), no Perry White, no Jimmy Olsen, no Daily Planet, no Kryptonite, no ability to fly, no Lex Luthor, etc.
In theory, if Siegel and Shuster's estates (when they're granted the rights in 2013) were to deny DC the right to exploit their trademark for 5 years, the estate could apply for the trademark to Superman, but if they wanted to do anything without DC, they'd have to pull a "Project Superpowers" and start from scratch, based only upon elements included in the original story.
Key words, "in theory".
It's unlikely they would pursue this, because any story elements too close to something DC already did (in a story DC still owns the copyright on) would probably land them in court.
Of course, if movie companies didn't insist on buying copyright extensions, Siegel & Shuster's estates wouldn't have a case, the Golden Age stories would be in the public domain where they belong, and DC would still control Superman as long as they maintained their trademark.
That's what they get for being greedy.
Speaking of which...
http://tinyurl.com/6ohd4w
Justice is served!