How to change background of Dashboard in Lion
The corresponding file seems to be located at

/System/Library/CoreServices/Dock.app/Contents/Resources/pirelli.png


It's a little piece of the pattern that's tiled: 

You can try replacing this with another tile of the same size, or even a larger image and see if it works! Then relaunch the Dock (from Activity Monitor or killall Dock) and it should show up.