I think at this point if you deal Pierce or especially KG, you should receive a player the caliber of Josh Smith in return.
What about this 3 way with Atl and the Clips?
Celts get Josh Smith, Lamar Odom and Anthony Morrow
Clippers get KG, Jason Terry and Leandro Barbosa (make salaries match)
Atlanta gets Eric Bledsoe, Caron Butler, Brandon Bass and a future draft pick (from the Celts.)
I think all 3 teams get value and for the Celts they also get to free up cap space by dealing KG, Bass and Terry.
And if Pierce opts out of his deal next year, even more cap space gets freed (think of a possible Rondo, Smith and Dwight Howard line up opening night 2013.)