[TheMilkCarton]

If you have fixed width boards (and Photoshop):

1) Open up a new photoshop file the same width as your boards (let's say 800px in this example)
2) New Layer > Fill it with white
3) Image > Canvas size > Increase the width to about 1000-1200px
4) Create a new layer under the 800px wide box and put a gradient on each side of it.
5) Once it looks good, Image > Canvas Size > and reduce the Height to 1px
6) Save for Web, and you're good to go.

Then for the actual CSS, in the Body box in Style Manager just add

url(/path/to/image.gif) top center repeat-y

to the "Background" box.

If you have fluid boards it's a little harder but not impossible.