It had been stated many times before that Carell would not be returning as Michael Scott, the role which he is most famous for. Just last week Executive Producer Greg Daniels told reporters, “I think Steve felt, and I agree, that the ‘Goodbye, Michael’ episode was his goodbye and he didn’t want to overshadow the ending that all the other characters deserved.”
Clearly the writers of the show don’t want the return of Carell to overshadow the ending for all of the other characters, hence all the denials thus far. Carell’s appearance looks as if it will just be a cameo and not much else, which makes me wonder why they would even bother to bring him back if he won’t be a big part of the final episode.