You have to keep in mind that trading Pierce would undercut our negotiating power with Bass, Green, Garnett, Allen, and Stiemsma. In my opinion, all of those players will be more expensive if the Celtics don't have Pierce because (1) they may take slightly less to stay together and (2) the Celtics wouldn't be able to use their limited cap space as a negotiation tactic.
I'm not saying that Pierce can't be traded, but I think it's important not to underestimate the hidden aspects of his value. In a vaccuum, I'd also consider Pierce for Gay. Considering all of the dominoes, though, I think it's a bad idea.