One thing more: See the first attached image in my second reply in this thread, and notice how the border is different from the background. Now this, how can be done?
Well, if the box will be a set width, you could create a one pixel tall by XX# pixels wide background image and repeat it down the y-axis behind the contents of the box. That would probably be the simplest way to do this.
You could also experiment with left/right borders and the box's background color. Just make sure this is located in the most buried <div>, and you'll need -1ox margin on either side so the corners overlap the borders beneath.
Either method will work well across browsers and will require no more code than what's been posted already. It'll simply take some experimentation to get it just right