Well, it's not a flat out "no."
Because it is inevitable whenever CM Punk opens himself up to a public forum, he did answer the question of whether or not a return to WWE is possible. He did not say no, but in continuing with recent trends, he seemed to hint that it would take the right offer.
Punk would say, "It'd have to be a very big bag."
During his appearance at Starrcast III, CM Punk would reveal that he is open to discussions with anybody as long as the right offer is made and presented in a professional fashion. He would also say that he is not interested in opening negotiations. Anybody with an offer would have to come to him.
He wouldn't answer much more about professional wrestling, but he would say in response to somebody asking how he would feel if he saw John Cena on the street, that he "probably wouldn't even see him"
Punk in recent interviews has also admitted to trying out for the FS1 show, WWE Backstage, but no further knowledge has come out about that tryout and if it will lead to anything.