I'm with WayOut; I don't get this, either from Boston's point or from Wallace's point.
San Antonio, Cleveland, Orlando (let's not forget about them), Dallas, and New Orleans are all better fits for Wallace. With the Celtics, he's an after-thought. Rasheed doesn't like playing center, and in Boston, that's what he'll be asked to do, unless he's filling in for Garnett, in which case, the Celtics are very, very ordinary. My thought was Orlando would be a great place for him; all of a sudden, you've gone from Nelson-Lee-Turkoglu-Lewis-Howard to Nelson-Carter-Lewis-Wallace-Howard - and that's a HUGE upgrade in size, rebounding, and defense (especially shot-blocking). And after losing Battie, the Magic need another big man. Wallace, who has 3-point range PLUS an inside game, fits the bill perfectly.
From Boston's point-of-view, you've now got an older Wallace taking minutes from a solid Powe and a developing Davis, who played exceptionally well at times during the playoffs. Development and integration of the younger guys - rather than potentially losing them as free agents - should have been the priority for the Celtics if they want to sustain success.
This is just a bad move by Wallace, and a high-risk, short-term move by Boston. I don't like it.