Fijate si esto te sirve:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
function ver(obj){
if(obj.value=='no' && obj.checked==true){
document.getElementById('observ').onfocus=function(){document.getElementById('observ').blur();}
}else{
document.getElementById('observ').onfocus=function(){}
}
}
</script>
</head>
<body>
<form action="" method="post">
si<input name="resp" type="radio" id="si" value="si" checked="checked" onclick="ver(this)" />
no<input id="no" name="resp" type="radio" value="no" onclick="ver(this)" />
<textarea name="observ" id="observ"></textarea>
</form>
</body>
</html>