Jake Marsh wrote this post a month ago about this topic of linen in the Notification Center versus linen in the Multitasking Tray:
Both of these interfaces [Notification Center and Multitasking Tray] have our friendly neighborhood linen as their background texture. On their own, this wouldn’t be too much of an issue but consider what Apple’s goals are with the linen texture. I would argue it is to establish a clear hierarchy of content and controls, from back to front. The linen was used early on as the “underneath” texture of
UIScrollViewsandUIWebViews.