I'm sure someone else has written a much more flexible categories plugin that will do this, but I haven't come across it yet. So I have tried my hand at building a WP plugin. Now I thought I'd share the results.
The plugin "EC List Categories" let's you view only the top-level (i.e., non-child) categories that have at least one published post. It makes displaying categories horizontally much easier, since there are no nested lists.
The plugin is available here: http://www.ecordero.net/index.php/archives/2004/08/17/5/
If you use it, I'd like to hear your feedback.