It has been confirmed that Carrie Fisher will definitely be back in the next Star Wars film. Fisher herself confirmed the news during an interview with Palm Beach Illustrated.
When asked if she could confirm that she would be reprising the role of Princess Leia, she replied, "Yes". She went on, joking about how she thought Leia would be portrayed. "Elderly. She's in an intergalactic old folks' home [laughs]. I just think she would be just like she was before, only slower and less inclined to be up for the big battle."
So, with this confirmation, we assume that Harrison Ford and Mark Hammill are coming back as well, but only time will tell.
Countdown to the release of Star Wars: Episode VII, with the iPhone Movie Countdown Clock App.