1) I think "nested" menus are utter inaccessible shite. Thus I would never have LI inside LI inside NAV. I'm really sick to death of flyouts, dropdowns, and all the other artsy fartsy navigational malarkey that just makes sites harder to use and often indicates sites with "too many pages for their own good" or "wah wah, but muh artsy design wuld geht farked up by a sidebar"

2) If you REALLY want it, use #top > nav > ul > li instead.