After a brief flirtation with pock marks, Apple embraced into the linen texture in a serious way with iOS 4, making it the background for both the multitasking fast app switcher dock, and for the open view of folders. Both of these UI metaphors were supposed to be "behind the scenes", with the iOS screen literally opening up to reveal them. With that was meant to convey the padding beneath the armor or the t-shirt beneath the jacket, who's to say? In the iOS 5 betas, however, Apple has gone all in on linen. While iOS 4 kept linen subtle and dark, iOS 5 brings lighter shades as well. You can find it as the background to the welcome screens when you update or re-install the OS. You can find it behind the new Notification Center when you swipe down from the title bar. Also, for the first time you can find it in OS X as well, in beta for Lion.
That makes it noticeable. Which is interesting. Texture is a Good Thing. It exists everywhere in nature and our eyes are used to it. Our eyes expect it. As long as it is consistent, it almost disappears, something we feel more than see in the overall richness of the environment around us. We'd notice if it wasn't there -- something would seem off or amiss -- but we shouldn't notice if it is there.
And that's where linen may be crossing the line for me -- I'm noticing it. Rather than a background that's contrasting and setting apart the foreground of the UI, rather than something my eyes ignore as they fall on the icons and buttons, it's becoming something my eyes fall on.
Though it hasn't come anywhere near that point yet, it does remind me of brushed-metal and the dark days of Apple design, before Leopard restored stately gray subtlety and consistency to the OS X UI. (So does the iCloud, LaunchPad, and associated conical gradients icons shown off at WWDC 2011, even more viscerally.)
It's possible I'm just noticing linen because it's new (though I'm not the only one). It's possible Apple will change the quality and quantity of it's use during the betas. Or it's possible linen is the new brushed-metal.