An interesting detail about the iOS 7 beta that Apple demonstrated at the 2013 WWDC keynote is that while the whole interface has depth and layers, there is no sign of that very common visual cue for depth: shadows.
I think there’s a simple answer to this, and it isn’t about iOS “going flat.” My impression of the illumination metaphor (for lack of a better word) in iOS 7 is that the whole user interface is backlit, just like the physical device screen is. Hence, no shadows.
This will initially be jarring to many because it is the exact opposite of what we are used to. We have been trained over many years to look at interfaces whose cues for depth come from a simulated ambient light from the front. This is the case not just in iOS before version 7, but all OS user interfaces. In short, the interfaces from yesteryear have not acknowledged that the true light source of those pixels come from the back, not the front.
One could argue that by making the user interface behave as if it is backlit, Apple is treating iOS 7 as a more integral part of the device itself. It’s not a mock front-lit interface with shadows and textures, it is a representation of the actual physical screen.
I don’t know how far or how consistently Apple’s designers are going to push this metaphor over the coming versions of iOS, or even the various beta versions of iOS 7 that we’ll see before its launch this fall, but color me intrigued.