The debug bar has a method ensure_ajaxurl() that resets the ajaxurl to admin_url('admin-ajax.php'); As BuddyPress uses wp-load.php as its ajaxurl, this causes some issues.
As Debug Bar can't be integrated with < BP 1.5.2, this isn't a problem yet. However, when 1.6 is released, BP will be re-integrating with the WordPress admin bar, and debug bar will be usable.
Perhaps an if ( ! is_plugin_active('buddypress') ) wrapper would do the trick.