That's actually a really good and balanced trade. The Cavs get someone capable of filling in the gap at small forward in Wallace, plus a reliable defender in Bradley (all while making them stronger contenders); in exchange, the Celtics get a mid-lateish first-rounder (14-18), but tons of cap space.
Only thing is this depends entirely on whether or not those rumors of the Cavs wanting to make a run at LeBron actually come to fruition. If so, they're going to want to keep that cap space, so they probably wouldn't be interested in Wallace (who also happens to play the same position as LeBron).