How to force one column layout on custom post type edit page?

There is a filter called get_user_option_meta-box-order_{$page} where $page is the name of the post type. Just make sure that submitdiv is the last value in the array:

add_filter( 'get_user_option_meta-box-order_post', 'wpse25793_one_column_for_all' );
function wpse25793_one_column_for_all( $order )
{
    return array(
        'normal'   => join( ",", array(
            'postexcerpt',
            'formatdiv',
            'trackbacksdiv',
            'tagsdiv-post_tag',
            'categorydiv',
            'postimagediv',
            'postcustom',
            'commentstatusdiv',
            'slugdiv',
            'authordiv',
            'submitdiv',
        ) ),
        'side'     => '',
        'advanced' => '',
    );
}