I've been doing a lot of work with Drupal recently. The documentation is somewhat unreliable on their site. I've found quite a few instances where there has been code that doesn't work or at least now in my version of drupal. If you're having issues with customising the search box as demonstrated in this article, try using this code instead in your theme (search-box.tpl.php):
<label for="search_theme_form_keys">Search</label><input type="text" maxlength="128" name="search_theme_form_keys" id="edit-search_theme_form_keys" title="Enter the terms you wish to search for." class="form-text" /><input type="submit" name="op" value="Search" /><input type="hidden" name="form_id" id="edit-search-theme-form" value="search_theme_form"/><input type="hidden" name="form_token" id="a-unique-id" value="<?php print drupal_get_token('search_theme_form'); ?>"/>
