This is a much better way to ask a question that I have asked that has not been answered! Thanks Josh!
Is it possible to have a single, global navigation that will display on all the sub sites within a multisite installation?
If you only make one theme available, and hardcode the navigation into the theme, yes.
Or something like this: http://dustyreagan.com/global-page-navigation-accross-wordpress-mu-blogs/
does this work with the twentyten theme?
You have to edit your themes in order to be able to achieve this effect, so yes, it’ll work in TwentyTen and every other theme. Using ipstenu’s link, you’d have to add that code in the functions.php file for each theme and then call <?php wp_list_main_pages(); ?> within the respective file (usually sidebar.php).
Could it also work to create a plugin that displays the menu on all sites via a custom template tag?
It could. You may need to experiment, but logically yes.
vvia a custom template tag?
That’s… pretty much what the example link have.
php function = template tag.