Cached part

This is very simple, a search-and-replace operation. Here is a list of changes:

• hook_menu() no longer takes any parameters (remove $may_cache).
• The value of path is the new index for $items.
• callback becomes page callback
• callback arguments becomes page arguments
• access becomes access callback and access arguments.

  For example, access => user_access('administer nodes') becomes 'access callback' => 'user_access', 'access arguments' => array('administer nodes').

  However, the default for 'access callback' is 'user_access' so you can leave that out.

  Complex access things must be moved to a function which can be called on runtime, user_is_anonymous and user_is_logged_in are useful helpers. See Access control for more.

• The title and description arguments should not have strings wrapped in t(), because translation of these happen in a later stage in the menu system. This allows translation of menu items to any language required on a site, adapting to the language used on the page.

Non-cached part

Let's suppose you had

/**
 * Implementation of hook_menu().
 */
function aggregator_menu($may_cache) {
  if (!$may_cache) {
    if (arg(0) == 'aggregator' && is_numeric(arg(2))) {
      if (arg(1) == 'sources') {
        $feed = aggregator_get_feed(arg(2));
        if ($feed) {
          $items[] = array('path' => 'aggregator/sources/'. $feed['fid'] .'/configure',
            'title' => t('Configure'),
            'callback' => 'drupal_get_form',
            'callback arguments' => array('aggregator_form_feed', $feed),
            'access' => user_access('administer news feeds'),
            'type' => MENU_LOCAL_TASK,
            'weight' => 1);
        }
      }
    }
  }

  return $items;
}

this becomes

/**
 * Implementation of hook_menu().
 */
function aggregator_menu() {
  $items['aggregator/sources/%aggregator_feed/configure'] = array(
    'title' => 'Configure',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('aggregator_form_feed', 2),
    // NOTE: as of Drupal 6.2, all menu items are *required* to have
    // access control.
    'access arguments' => array('administer news feeds'),
    'type' => MENU_LOCAL_TASK,
    'weight' => 1,
  );

  return $items;
}

If we would use just a percent sign aggregator/sources/%/configure it'd match everything, including non-numeric values as well, like aggregator/sources/foo/configure. However, with %aggregator_feed we ask for a feed to be loaded based on the third argument and anything that's not a valid feed will lead to a 404.

About the use of wildcards, foreach() loops and MENU_ITEMs

In Drupal 5, we used to use foreach() loops to recursively declare several MENU_ITEMs or MENU_SUGGESTED_ITEMs.

In Drupal 6, we only need one entry in hook_menu() with the use of a proper wildcard. Additionally, you can create as many menu items (enabled or not) as you see fit with menu_link_save().

For a detailed discussion on this topic, see this issue.

Additional Run Once Code

Non menu code that was placed in hook_menu !$may_cache so that it could be run during initialisation, should now be moved to hook_init. Previously we called hook_init twice, once early in the bootstrap process, second just after the bootstrap has finished. The first instance is now called boot instead of init.

If you were using a loop in your menu code then you likely need menu_link_save.

A horribly complex example can be found in the menu of http://drupal.org/files/issues/akismet.d6-port.patch. It involves just about every possible path manipulation trick one could ever need for the path akismet/%akismet/%/'. $op, and then some. It passes the real page callback in $map[0] which is a bit of a hack, but is also quite useful.