Is there any way to restrict access to a Unity Distribution List to certain users only? A sort of "private group" distribution list?
Not with Unity - can can restrict users from addressing to _any_ public distribution lists but you can't allow them access to some public dls and not others. Some sites get cute with non spellable names and assigning extension numbers only some folks know for addressing by ID but this is a little wonky.
Unity Connection allows this type of configuration using search spaces (i.e. you assign DLs to a partition which is assigned to search spaces just like any other objects - users are limited to objects in their search space when addressing messages).