The Kirbys seek to win the copyrights back to Hulk, Thor, Iron Man, X-Men, Fantastic Four, etc., and claim some (or all) of the profits. If they win back the copyright, it wouldn't kick in until 2014 so presumably they wouldn't be able to claim the "back profits" the characters have made, but anything going forward would be a problem. As Bleeding Cool notes, Disney and Marvel will always own the trademarks, and the movie studios would probably also be safe in their acquisitions, but continuing to publish anything Kirby created would be problematic.
The attorney Marc Toberoff declined to comment on the case, but Disney released an official statement that shrugged off the lawsuit, noting "the notices involved are an attempt to terminate rights 7 to 10 years from now, and involve claims that were fully considered in the acquisition." This will undoubtedly be a case fraught with as many tangles as the Siegel and Shuster suit was. Will Marvel end up owning Iron Man's armor, but the Kirbys will own Tony Stark and Pepper Potts? Will it create problems for Marvel Studios down the line? We shall watch and see.