WP3.1 : custom post types templates

Hi, i’ve read there will be a new feature to have specific templates for custom post types; but I wasn’t able to find good explanations.
in my code, i have

register_post_type( 'yclad'
...
'rewrite' => Array( 'slug' => _x('classified-ads','slug','yclads'), 'with_front' => false ),
'has_archive' => true

So If I well understood, with this;

1. /classified-ads/ will be the rewritten url for this post type.
2. archive-yclad.php should be the template loaded when accessing /wordpress/?post_type=yclad

Is that correct ?

BTW,
/classified-ads/my-post works; but
/classified-ads/ goes to 404.

I did try to refresh the permalinks, and also to put

flush_rewrite_rules( false );

just after

register_post_type(...

It does not work.

Also, is there a conditionnal tag to check if we are browsing a particular post type ?
I would like a function to detect if we are on /wordpress/?post_type=yclad

Is there something like is_archive(‘yclad’) ?