<form name="archive_pulldown">
<select name="archive_option">
<option>月別アーカイブ</option>
<?php wp_get_archives('format=option'); ?>
</select>
<input type="button" value="GO" onClick="window.location = (document.forms.archive_pulldown.archive_option[document.forms.archive_pulldown.archive_option.selectedIndex].value);" />
</form>