og in selectlist at the registration form

Hi,
I achieved that by hacking the "og.module" file in the og module directory,
Look for this section:
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function og_user($op, $edit, &$account, $category = NULL) {
  global $user;

  switch ($op) {
    case 'register':
      $options = array();
      list($types, $in) = og_get_sql_args();

      // If groups are passed on querystring, just use them.
      if (isset($_REQUEST['gids']) && $gids = $_REQUEST['gids']) {
        $default_value = $gids;
        foreach ($gids as $gid) {
          $nids[] = (int)$gid;
        }
        $in_nids = db_placeholders($nids);
        $sql = "SELECT n.nid, n.title, o.* FROM {node} n INNER JOIN {og} o ON n.nid = o.nid WHERE n.type $in AND n.status = 1 AND n.nid IN ($in_nids) ORDER BY n.title";
        $result = db_query(db_rewrite_sql($sql), array_merge($types, $nids));
      }
      else {
        $default_value = array();
        // perhaps this should be a View
        $result = db_query(db_rewrite_sql("SELECT n.nid, n.title, o.* FROM {node} n INNER JOIN {og} o ON n.nid = o.nid WHERE n.type $in AND n.status = 1 AND o.og_register = 1 ORDER BY n.title"), $types);
      }

      while ($group = db_fetch_object($result)) {
        $options[$group->nid] = '<span class="og-registration-'.$group->nid.'">'. t('Join %name.', array('%name' => $group->title)). "</span>\n";
        if ($group->selective) {
          $options[$group->nid] .= ' '. t('(approval needed)');
        }
      }

      if (count($options)) {
        $form['og_register'] = array('#type' => 'fieldset', '#title' => t('Groups'));
        $form['og_register']['og_register'] = array(
          '#type' => 'checkboxes',
          '#options' => $options,
          '#default_value' => $default_value,
        );
        return $form;
      }

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And replace it with this:

--------------------------------------------------------------------------------------------------------

function og_user($op, $edit, &$account, $category = NULL) {
  global $user;

  switch ($op) {
    case 'register':
      $options = array();
      list($types, $in) = og_get_sql_args();

      // If groups are passed on querystring, just use them.
      if (isset($_REQUEST['gids']) && $gids = $_REQUEST['gids']) {
        $default_value = $gids;
        foreach ($gids as $gid) {
          $nids[] = (int)$gid;
        }
        $in_nids = db_placeholders($nids);
        $sql = "SELECT n.nid, n.title, o.* FROM {node} n INNER JOIN {og} o ON n.nid = o.nid WHERE n.type $in AND n.status = 1 AND n.nid IN ($in_nids) ORDER BY n.title";
        $result = db_query(db_rewrite_sql($sql), array_merge($types, $nids));
      }
      else {
        $default_value = array();
        // perhaps this should be a View
        $result = db_query(db_rewrite_sql("SELECT n.nid, n.title, o.* FROM {node} n INNER JOIN {og} o ON n.nid = o.nid WHERE n.type $in AND n.status = 1 AND o.og_register = 1 ORDER BY n.title"), $types);
      }

      while ($group = db_fetch_object($result)) {
        $options[$group->nid] = $group->title;//Modified
        if ($group->selective) {
          $options[$group->nid] .= ' '. t('(approval needed)');
        }
      }

      if (count($options)) {
        $form['og_register'] = array('#type' => 'fieldset', '#title' => t('Groups'));
        $form['og_register']['og_register'] = array(
          '#type' => 'select',//Modified
          '#options' => $options,
          '#default_value' => $default_value,
        );
        return $form;
      }

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I modified the field type used to present groups on registration form from "checkboxes" to "select"
Then, I prevented the printed variables from being themed.
This has the advantage of presenting groups on registration form in a compact way, and prevent user from subscribing to more than a group when it's needed.
Hope this helps someone.