4 finger swipe already exists on OSX as a system wide gesture for Expose and Application switching. No, it's not exactly an apples to apples comparison as OSX doesn't depend on gestures as heavily, but the tradeoff is well worth the benefit.

You no longer need to push on a physical button to switch between apps!

Re: the container peeking through the application's interface.

This isn't necessarily a bad thing either. I'd argue that consistency can (in some cases) be more important than giving application developers total control. Have you ever been to a site which intercepts right clicks so that you can't copy and paste?

I don't know, I think I'd rather have a physical button that I can press with the hand I hold the phone with than have to do a four-figured operation on the screen.

I don't think the four-finger operations are on the iPhone/iPod, I think they're only on the iPad.

Correct. At least, that's the info that's currently available. :) If you try fitting 4/5 fingers on the phone, it's way too small to work well anyway. I don't expect them to show up there. If they do remove the button, I'd expect to see it only removed from the iPad.

I feel the number of fingers used in a gesture maps well to the 'scope' of the gesture, that's how I have BetterTouchTool set up. I use two-finger gestures for things like scrolling and zooming (content-level), three finger gestures for things like back, forward, next tab, previous tab (window- or app-level) and four fingers for expose/task switcher/hide all windows (multi-app/OS level).

There already are global gestures for VoiceOver and zooming.

You can disable/have to enable them, but only globally. I do not know whether applications can be programmed to ignore or override these behaviours.

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.

The three-finger zooming I refer to (Settings-General-Accessibility-Zoom) is global.