[jquery] jQuery - Disable Form Fields

<script type="text/javascript" src="jquery.js"></script>          
<script type="text/javascript">
  $(document).ready(function() {
      $("#suburb").blur(function() {
          if ($(this).val() != '')
              $("#post_code").attr("disabled", "disabled");
          else
              $("#post_code").removeAttr("disabled");
      });

      $("#post_code").blur(function() {
          if ($(this).val() != '')
              $("#suburb").attr("disabled", "disabled");
          else
              $("#suburb").removeAttr("disabled");
      });
  });
</script>

You'll also need to add a value attribute to the first option under your select element:

<option value=""></option>