I wrote this nifty function to filter select boxes when their value is changed...

$.fn.cascade = function() {
    var opts = this.children('option');
    var rel = this.attr('rel');
    $('[name='+rel+']').change(function() {
        var val = $(this).val();
        var disp = opts.filter('[rel='+val+']');
        opts.filter(':visible').hide();
        disp.show();
        if(!disp.filter(':selected').length) {
            disp.filter(':first').attr('selected','selected');
        }
    }).trigger('change');
    return this;
}

It looks at the rel property, and if the element indicated by rel changes, then it filters the list to only show the options that have that value... for example, it works on HTML that looks like this:

<select id="id-pickup_address-country" name="pickup_address-country">
  <option selected="selected" value="CA">Canada
  </option>
  <option value="US">United States
  </option>
</select>

<select id="id-pickup_address-province" rel="pickup_address-country" name="pickup_address-province">
  <option rel="CA" value="AB">Alberta
  </option>
  <option selected="selected" rel="CA" value="BC">British Columbia
  </option>
  <option rel="CA" value="MB">Manitoba
  </option>...
</select>

However, I just discovered it doesn't work properly in IE (of course!) which doesn't seem to allow you to hide options. How can I work around this?