How can I increase the post count for custom post types only?

I think something like this should work:

add_action('parse_query', 'wpse32932_parse_query'); 
function wpse32932_parse_query( $wp ){
    if( $wp->is_post_type_archive ):
        $wp->query_vars['posts_per_page'] = 30;
    endif;
    return $wp;
}