See if http://drupal.org/project/cacheexclude can do it for you. If not you need to create a module that will implement hook_init. Inside of that hook you need to load the node from arg() from there see if the node is of a certian type; if it is disable the page cache.

    <?php
      // Get the current active node object if present.
      $node = menu_get_object();
      if (!empty($node) && $node->type == 'page') {
        drupal_page_is_cacheable(FALSE);
      }
    ?>