Shortstop11, Did you watch the Home Run Derby and see the super slow motion swings? In every swing, the replay showed the bat striking the ball at an upward angle, an angle close to the flight of the ball. The bat usually started it's upward trajectory a good foot and a half before striking the ball. And the hitters were not changing their swing for the derby, in fact David Wright seemed to use much less of an uppercut than he uses in a game.
As for MLB hitters saying that they swing down, many of them say that they feel doing it, but slow motion replays reveal that they do not. Try this website and tell me who swings down...
Play these clips frame by frame, and stop each clip the frame before contact. You will never see a bat going down and "compressing" through the ball. Despite what Arod and Jeff Cirillo say.