Ciao morphy79,

JavaScript è CaseSensitive... readOnly va con la O Maiuscola
codice:
<form>
<select name="NomeSelect" onchange="ro=(this.selectedIndex==1);this.form.NomeText1.readOnly=ro;this.form.NomeText2.readOnly=ro;this.form.NomeText3.readOnly=ro;">
  <option>Normale</option>
  <option>readOnly</option>
</select>
<input type="text" name="NomeText1">
<input type="text" name="NomeText2">
<input type="text" name="NomeText3">
</form>