While I don't see the Celtics being much of a better destination for Big Z, I do wonder if he thinks twice about rejoining the Cavs. There are not enough minutes to go around for Shaq, Jamison, Big Z, Hickson, and Varejo. In fact, come playoff time, there might really only be significant minutes for 3 of them, with spot minutes for a 4th.
Now if Big Z's only goal is to get a ring, even if he doesn't contribute, then Cleveland might be a good place to go. However, if he wants a shot at a ring where he actually plays, he might be better suited going elsewhere.
I don't see the C's as being much better, with our glut of KG, Perk, Wallace, and BBD. I also am not sure that he's the right fit (you think Wallace is slow rotating defensively?). However, he might be better suited on a team where he's undoubtedly the backup center.