[Deff]

Многострочный Вариант

<div style="background-color:#fff">

<form id="form" name="form">
  <select name="spisok" id="spisok" style="width: 155px;" onchange="Onchange(this.value);return true;">
     <option selected="selected" value="0"></option>
     <option value="1">1-я строка списка</option>
     <option value="2">2-я строка списка</option>
     <option value="3">3-я строка списка</option>
  </select>

  <span id=span>
     <input id="pole1" type="text" value="1" style="width:30px;position:absolute;z-index:-100;"/>
     <input id="pole2" type="text" value="2" style="width:30px;position:absolute;z-index:-100;"/>
     <input id="pole3" type="text" value="3" style="width:30px;position:absolute;z-index:-100;"/>
  </span>

</form>

</div>

<script type="text/javascript">
function Onchange (c) {
  var a=document.getElementById('form')
  var elems=a.getElementsByTagName('input')
  for(var i=0; i<elems.length; i++) {
    elems[i].style.zIndex = '-100';
  }

     a=a.document.getElementById('pole'+c);
     a.style.zIndex = '12';
     a.focus();
}
</script>