Zooming isn't necessarily defined as a system-wide gesture, but more of a convention. I believe the rules say that your app will get rejected if you use a pinch

Zooming isn't necessarily defined as a system-wide gesture, but more of a convention. I believe the rules say that your app will get rejected if you use a pinch gesture for anything but a zoom-like operation, but it isn't enforced by the system because there could be other valid reasons for having two fingers on the screen moving in a way that might seem like a pinch gesture if viewed out of context. These new iPad gestures cannot be overridden by your app because the OS will cancel touch events out from under your app and take them for itself when it detects the gestures are in progress. For VoiceOver (which I've not spent a ton of time investigating yet), it seems the app doesn't even get the touch events until a VoiceOver action is "committed" in some way. VoiceOver basically overlays the entire screen with an invisible window and intercepts all touches, so it basically redefines how apps work and defines a set of its own gestures regardless of what you might have defined in your app.