[brownleej]

I don't think that will be possible. From the video, it looks to me like the three-finger gesture is only for switching between apps, and having an app intercept it would be jarring. For instance, if you were swiping through your recent apps, and got to the browser, you wouldn't be able to move on using the same gesture, and it would be like hitting a brick wall. In fact, I imagine that individual apps won't even be able to capture these gestures, because the system will catch them first, and interpret them as app-switching.

[pieter]

From what I understand, switching apps is done using 4 or 5 fingers, so the 3 finger gesture is still free to be used by the app itself.

[BigZaphod]

I believe 3 finger gestures are used by VoiceOver so there's a possible conflict there, but VoiceOver is a system wide on-off setting so I don't know if that'd be a good enough reason not to use them as shortcuts. VoiceOver more or less redefines how you interact with apps anyway, so I'm not sure it'd bit a big problem, but Apple may want to avoid having too many different actions for different combinations of fingers.