In 10.4, when you press the '+' symbol in the bottom left corner while Dashboard is active, the Dashboard shelf slides up into view containing all your widgets. The background image used for the shelf can be found here:

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

You can replace this image with any PNG image you like and it'll be tiled, or you can use a single, full width image. The shelf is 118 pixels high and as wide as your monitor (1024 pixels in my case), so you can use those dimensions to construct your own shelf background. Also in the same directory are the images for the next and previous arrows on the shelf.

Go be creative!

[robg adds: As seen below, I replaced this image with a non-perforated gradient background, and I'm now a much happier Dashboard user. It's not that I mind the perforations all that much, but they were a bit too strong for regular viewing.


The bottom area (where the widget names appear) will need to be lighter in color for best results, as the text is both light and shadowed. The height of the text area is 14 pixels. Note also that you do not need to make your PNG file the full width of your monitor -- the image will repeat horizontally if it's not full-width. I used a tiny 2KB PNG to create the above full shelf background.]