Update: I have a new article that supports nested menus with instructions on use at Select tag overlap in IE, Part II. Please go to the new article as it give you information that is not available in this article.
Sometime ago I was asked if I knew of an IE fix to hide select elements when postitioning other elements over them.
The problem only occurs in IE when the dropdown menu overlaps a select tag in form. It appears that nother will override the z-index of the select tag in IE exept to hide it.
UPDATE: You see a temporary sample with opacity of a single drop down menu here.