Manca la proprietà style.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<form name="inp_db">
<select name="cat_comp" onChange="nascondi_inp_txt();" class="inputform" id="cat_comp" onfocus="this.style.background = '#EDEDED';" onblur="this.style.background = '#F7F7F7';">
                      <option value="Garanzia" selected>Garanzia</option>
                      <option value="Sistema_Operativo">Sistema Operativo</option>
                      <option value="Colore">Colore</option>
                      <option value="Alimentatore">Alimentatore</option>
                      <option value="Cable_Management_System">Cable Management System</option>
                      <option value="1">- Campo Opzionale -</option>
                    </select>

<script language="javascript">
function nascondi_inp_txt(){
if (document.inp_db.cat_comp.value == '1'){
  document.getElementById('inp_text').style.visibility='visible';
 }
  else
 {
  document.getElementById('inp_text').style.visibility='hidden';
}
}
</script>
<span id=inp_text>
 <input name="pippo" type="text" class="inputform" id="pippo" onfocus="this.style.background = '#EDEDED';" onblur="this.style.background = '#F7F7F7';">
</span>
</form>								
</body>
</html>