The product team has released the code for the menu control used in WSS v3 and MOSS 2007. This menu class was previously sealed and therefore developers could not derive their own control and override just some of the behavior. The class in the install bits is actually named AspMenu. The code released is under a new name MOSSMenu. With this code, you can more easily see the inner workings of how the menus are created and have a starting point for creating your own.

http://blogs.msdn.com/ecm/archive/2006/12/02/customizing-the-wss-3-0-moss-2007-menu-control-mossmenu-source-code-released.aspx