Se ho capito bene...

codice:
...
<head>
<script>
function show(field) {
  if (field == 'rb1') {
    document.getElementById('rb1text').style.display = 'inline';
    document.getElementById('rb2text').style.display = 'none';
  }
  else {
    document.getElementById('rb1text').style.display = 'none';
    document.getElementById('rb2text').style.display = 'inline';  
  }
}
</script>
</head>

<body>
....
<input type="radio" name="rb" id="rb" value="rb1" checked="checked" onclick="show(this.value)" value="campo1" />
<input type="text" name="rb1text" id="rb1text" />


<input type="radio" name="rb" id="rb" value="rb2" onclick="show(this.value)" value="campo2">
<input type="text" name="rb2text" id="rb2text" style="display:none" />

Ciao