...and here it is: http://axure.com/CS/blogs/axure/archive/2006/10/25/Creating-MouseOver-Flyout-Menus-using-Dynamic-Panels.aspx

This article walks through the steps to create dynamic or flyout menus in Axure RP that appear when the mouse is moved over a menu item and disappear when the mouse is moved away from the menu. This particular example will mimic a menu similar to what you would see in a typical desktop application. Here is the resulting RP file and ...