Thank you!! That worked great.
In the custom block options for the Navigation menu, you'd be able to use PHP code to determine whether to actually display it. Something like this would tend to work (untested):
<?php global $user; return $user->uid ? TRUE : FALSE; ?>
What this is saying is:
load in the special Drupal object $user.
if the user's user ID exists ($user->uid), then it is TRUE that we want to display this block. If it doesn't exist (in the case of an unauthenticated viewer), then it is not true (FALSE) that we want to display the block.
- jody