So you want the corners to appear on hover. Can’t you just do:
#nav:hover { background:url(top.jpg) no-repeat;}
#nav ul:hover {background:url(bottom.jpg) no-repeat bottom left;}
The containing div would have an id of nav.
This won’t work in ie6, but there are various work arounds. I usually use the JS fix as IE6 is normally low on my supported browser list.
