Yes, there seems to be a dearth of documentation on the new custom post types. I'd like to know how to add a new post type and then not only how to select just that post type but also how to choose to exclude those posts from other functions, such as RSS feeds, site searches, home page, archives, etc.
I'm thinking I might want to write a very simple plugin for discussion forums that use the standard WP tables. The benefits would be that the forums could automatically show in site navigation, standard widgets for posts/pages, site searches; automatic comment and pingback capability; etc.
I did find just one reference to this in wp-includes/posts.php there is a call to a function register_post_type(). However, doing a quick file search, I can't find where that function itself is defined. Of course many standard WP functions have always allowed a post_type parameter, so I assume they have been rewritten to validate posts types against those that are officially registered with register_post_type() so not much change is needed in selecting the custom post types.
I have also read that the "user interface" for custom post types is not coming until WP v3.0. I'm not really sure what that means, although I guess it probably means there will be more specialized functions for selecting, displaying, updating custom post types just to make things easier.