I am having an issue with my nav not appearing on a sub blog when another plugin is enabled (Role Scoper) and I just wan’t to be sure that the Multi-level Navigation Plugin is compatible with WPMU.
You shouldn’t use it with WP Mu if you are running a multi-user setup. It should run okay (I assume) if you are just using Mu to power multiple sites from the one installation.
The latest beta of the PixoPoint Menu plugin should work fine in multi-user setups, but I’ve never tested it.
You could hack one blog from another blog via the Multi-level Navigation plugin. The plugin should work, but I never built it with Mu in mind. All of my newer plugins are built with this extra layer of security in mind, but it never occurred to me that it was necessary when I made the Multi-level navigation plugin.