This rumor comes from Mark Hamill's friend and former roommate Robert Englund, who is best known for playing Freddy Krueger in the Nightmare on Elm Street series...
From Comic Book:
While appearing at the Nashville Comic Expo, Robert Englund talked about how both he and Hamill have done a lot of voice work for animation. He also let slip out a big secret in regards to exactly what Hamill is up to currently. “Mark now—they’ve got Mark in the gym, because Mark’s coming back as Luke Skywalker. They’ve got him doing his sit ups,” said Englund.
This story fits with the rumor from few months ago about personal trainers being hired for the Big Three.
Personally I have no doubts that some or all three of them will appear in Episode 7. Remember what George Lucas said to businessweek.com back in march?
Asked whether members of the original Star Wars cast will appear in Episode VII and if he called them before the deal closed to keep them informed, Lucas says, “We had already signed Mark and Carrie and Harrison—or we were pretty much in final stages of negotiation. So I called them to say, ‘Look, this is what’s going on.’ ” He pauses. “Maybe I’m not supposed to say that. I think they want to announce that with some big whoop-de-do, but we were negotiating with them.” Then he adds: “I won’t say whether the negotiations were successful or not.”
Of course things could change but I still trust good old George. :)