Based on comments from Pierce and KG, I have doubts that Pierce would be willing to play alongside Ray Allen again.
After the things Ray said about Boston, I also have my doubts that Ainge would even consider signing him - especially since he left us hanging in free agency and went to the Heat (who eventually knocked us out of the playoffs).
Everybody puts up the professional / business front, but after winning a title together and battling together all those years, I can't imagine a move like that would be something you'd easily forget.