Where can I find the gray linen texture that's used as the Lion login screen background?
Open System Preferences » Desktop and Screen Saver:
Click the "+" at the bottom left:
Press Command + Shift + G to bring up the "Go To Folder" menu, and paste in the path to the background image:

/System/Library/Frameworks/AppKit.framework/Versions/C/Resources/NSTexturedFullScreenBackgroundColor.png


(Press Go, and then Return  when you see the NSTexturedFullScreenBackgroundColor.png file is selected)
You will now be back at the System Preferences window. Make sure that the drop-down selector is set to 'Tile'


Voilà!