Formulaire dynamique

<div id="regions">
    <select name="region" id="regions_france" class="invisible">
        ...
      </select>

      <select name="region" id="regions_canada">
          ...
      </selet>

      ...
</div>

.invisible { display: none; }

<select name="pays" id="pays">
    <option value="">Votre pays :</option>
    <option value="france">France</option>
    <option value="canada">Canada</option>
    ...
</select>

<script type="text/javascript">
window.onload = function() {
    document.getElementById('pays').onchange = function() {
        /* On masque tout */
        var listes = document.getElementById('regions').getElementsByTagName('select');
        for(i = 0 ; i < listes.length ; i++) {
            listes[i].style.display = 'none';
        }

        /* On affiche la bonne liste */
        var liste = document.getElementById('regions_' + this.value);
        if(typeof(liste) !== "undefined") {
            liste.style.display = 'inline';
        }
    }
};
</script>