Ein Versuch für WP 2.1:
PHP
<form id="archiveform" action="">
<select name="archive_chrono" onchange="window.location =
(document.forms.archiveform.archive_chrono[document.forms.archiveform.archive_chrono.selectedIndex].value);">
<option value=''>Select Month</option>
<?php wp_get_archives('monthly','','option'); ?>
</select>
</form>