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He says the "word on the street" is that engineers inside Apple who are authorized to carry around new iPhones have a filter on their screen so that the side viewing angles are diminished. (Think about one of those filters people put on their desktop screens at work.)
The filter on the iPhone screen makes it hard for people to see "the apparently rather significant system-wide UI overhaul."
Rene Ritchie, another well-sourced Apple writer adds, "[Jonathan] Ive’s work is apparently making many people really happy, but will also apparently make rich-texture-loving designers sad."
In other words, Ive, who was promoted to lead the design of Apple's software, is wiping out stuff like fake stitching and fake wood textures that often pop up in iOS.
Gruber also says he's hearing that Apple is running behind on iOS 7 and has pulled engineers working on other projects to get iOS 7 finished on time.
His comments were originally made on Branch, a commenting platform. There's a bunch of Apple writers gabbing about the latest they're hearing on iOS.