Got it. I had added Justin Tadlock's code to my functions.php to make the homepage show all content types. It did. But his code also broke CMS Press' templating. (I tested it by adding some static text to the index.php, and confirmed that index.php WAS being used for both domain.com and domain.com/cars/ and it ignored completely the index-cars.php template). His code:
add_filter( 'pre_get_posts', 'my_get_posts' );
function my_get_posts( $query ) {
if ( is_home())
$query->set( 'post_type', array( 'post', 'cars' ));
return $query;
}
So, in short, your plugin is A-Okay. Great, actually! It's much better than Custom Post Type UI, in my opinion, b/c you allow URL's to end in .html. CPT UI forces you to end in a trailing slash, which I loathe, and doesn't fit with my existing permalink structure. So I switched to CMS Press!
BTW, while we're talking, do you have a way to display all content types on the homepage and in the main feed?