I actually think San Antonio might do it for a shot of energy and to get Jefferson out of the way to speed up rebuilding. Maybe you guys haven't been watching but (a) the Spurs aren't that good at all and haven't even faced the tough part of their schedule yet, (b) Manu Ginobli is far more done than Ray and gone after this season and (c) Richard Jefferson sucks or at least has been an absolute bust in San Antonio.
But there's no reason for the Celtics to do it for reasons (b) and (c) above. We actually would have less of a chance than we already do this year if we make this trade, and it sure doesn't help us long term.
The other thing - on the Spurs side, they really have no need for Davis. Blair is already a better player, and cheaper. No need for another relatively undersized power forward, particularly one who can't rebound or finish nearly as well as the one they already have.