The docs say that CSS opacity should work. I'm changing a div opacity using:

#listing {
opacity:0.2
}

Looks good in Firefox 3.6, but the PDF does not show any opacity.

Are there some specific rules I should follow with opacity? Or another method of getting this div to "fade into the background"?

Unfortunately, the docs are lying! You can use rgba() though, and stroke-opacity and fill-opacity in SVG.