I tried - couldn't get it to work

The problem is because in IE 6 you can't have li:hover and have to use li a:hover, which leads to worse problems because you have to hack IE 6 so that the next ul looks like it is inside of the a tag (this is all your if IE 7 conditionals in the html). But with that it was unclear to me what css could do properly. I realize you want a css dd, but I would just remove all of your IE 7 conditionals and add some IE 6 ones that add javascript. It's ugly but it will work. I may look at it some more. If I get somewhere I'll reply here.
Also, I suggest adding position:relative to your #menu_container style (the z-index isn't working without that - you can tell on the far left dd on the lowest link).
Hope you get this working!