Hi @Scott B:
Your example doesn’t work for me when I use if (is_home()) but does when I use if (is_front_page()). Is there any chance that is the problem?
Other than that it seems to work fine on my WordPress v3.0.3 install. Do you have other plugins that might be disabling the output of wp_list_categories()?
P.S. I’m assuming you are using add_action( 'init','cb_category_listing'); just for debugging and not as part of your plugin?