How can I replace the 'Linen' that is prevalent in many Lion apps?
The linen backdrop file is
/System/Library/Frameworks/AppKit.framework/Versions/C/Resources/NSTexturedFullScreenBackgroundColor.png

You can edit that file, or replace it with a file of the same name, but be certain that the replacement file has a resolution of 72 pixels per inch or you probably won't like the result.

ᔥ John Martellaro: OS X Lion: Change Login Screen’s Dirty Linen

Certain applications and situations draw their background from different sources. for example, the background file for Mission Control can be found at /System/Library/CoreServices/Dock.app/Contents/Resources/defaultdesktop.png

If you replace that file, you'll need to do a killall Dock command in the terminal before the change will take effect.

ᔥ OS X Daily: Change the Mission Control Background Wallpaper Image in Mac OS X Lion