Mark Hamill has confirmed that he is considering appearing in a new Star Wars film that is currently being developed by Disney, which purchased Lucasfilm for more than $4 billion in 2012.  In a recent interview Hamill recounts that "They’re talking to us.  George (Lucas) wanted to know whether we’d be interested.  He said that if we didn’t want to do it, they wouldn’t cast other actors in our parts, they’d just write us out.  I can tell you right away that we haven’t signed any contracts.  We’re in the stage where they want us to go and meet Michael Arndt, who is the writer, and Kathleen Kennedy, who is going to run Lucasfilm.  Both had meetings set that were postponed--on their end, not mine.  They busier than I am."
So no deals have been struck, but it does appear that the new Star Wars film will have a plotline that could at least accommodate "cameos" or perhaps more from the actors who helped make the original trilogy such a success.  Rumors had already connected Harrison Ford, Carrie Fisher, and Hamill to the new project (see "A Writer for 'Star Wars Episode 7'").  But how many members of the original cast will make the transition to Star Wars 7?  
Hamill told ET Online that he wanted "to go back to the way it was, in the sense that ours (the original trilogy) was much more carefree, lighthearted and humorous... and another thing I’d want to make sure that we would have the whole gang back.  Are Carrie, Harrison, Billy Dee, and Tony Daniels, everybody that’s still around from the original returning?"
The number of original cast members who will return, and the extent and nature of the roles that they would play in the new Star Wars film remains to be seen, but the powers that be at the new Disney-run Lucasfilm do appear to be interested in bringing some of the principals for the original trilogy back for Star Wars 7.