I was looking for a weird (don't ask why) background for my desktop, and I thought of the pattern for Dashboard's widget bar (a Mac Pro-like perforated metal strip). How could I make this into a uniform desktop pattern? The answer lies in /System » Library » Dock.app » (Control-click and pick Show Package Contents from the pop-up menu) » Contents » Resources.
First, I located perf.png in that folder, and dragged it to my desktop. Then, I opened that copy in Preview. I cropped this to 10x10 pixels, starting from the top hole with a one-pixel left margin, and ending below the hole to right (see example at right).
(On certain computers, for some reason, there is a different bottom to the hole. Leave a margin of one pixel if the hole is 3x4, and no margin if it is 3x5.) Save the file as a PNG, and then set it as your desktop picture with the Tile option. Result? Full metal desktop.
[robg adds: The image above is from my machine, and it's actually 9x10, but it worked in my testing...though I reverted to my rotating set of desktop pictures after testing.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091229084419214